ASCPprep.com | Clinical Chemistry
Chapter 12 β€” Diabetes Mellitus
ASCP Board Exam Β· Lab-Focused Study Guide
Methods Β· Clinical Interpretation Β· Pre-analytical Variables Β· Practice Questions

Section 1 β€” Priority Table & Quick Foundation

Priority Table β€” What the ASCP Tests Most

Topic Priority Why It’s Tested
Diagnostic criteria (FPG, OGTT, HbA1c, random) HIGH Numbers are testable, repeatable, and the cornerstone of every clinical scenario.
HbA1c β€” meaning, methods, interferences HIGH Reflects 8–12 weeks of glycemia; tested for hemoglobinopathy and RBC lifespan traps.
Glucose specimen handling (glycolysis, fluoride, citrate, ice) HIGH Classic pre-analytical trap β€” wrong tube β†’ falsely low glucose β†’ wrong diagnosis.
Type 1 vs Type 2 distinction (C-peptide, autoantibodies) HIGH C-peptide & GAD65/IA-2/ZnT8 are favorite differentiators on case stems.
Gestational Diabetes (GDM) β€” one-step 75 g, two-step 50β†’100 g HIGH Cutoffs for fasting/1h/2h/3h are highly testable.
Ketone bodies β€” Ξ²-hydroxybutyrate vs acetoacetate HIGH Nitroprusside trap: misses Ξ²-OH-butyrate; result “worsens” with treatment.
C-peptide β€” surreptitious insulin vs insulinoma MEDIUM Classic hypoglycemia work-up scenario.
Albuminuria (ACR) screening for diabetic nephropathy MEDIUM First morning sample, 3 specimens, ACR trap on dilute urine.
Glucose meters β€” performance, hematocrit, oxygen interferences MEDIUM POC limitations β€” never used to diagnose.
Counterregulatory hormones (glucagon, epi, cortisol, GH) MEDIUM Hypoglycemia and stress hyperglycemia questions.
Fructosamine, glycated albumin, 1,5-AG LOW Useful when HbA1c is invalid; rare on the boards.
Insulin synthesis pathway / proinsulin processing LOW Generally too detailed for ASCP MLS.
KEY POINT β€” Where to spend your time
Master the diagnostic numbers (FPG 126, 2-h OGTT 200, HbA1c 6.5%, random 200 with symptoms), the pre-analytical glucose tube logic (gray-top sodium fluoride, ice slurry, citrate buffer), and the HbA1c interferences (hemoglobinopathies, shortened RBC lifespan). Skim the deep biochemistry of insulin signaling and the laundry list of GLUT transporters.

Quick Foundation β€” From Zero in 8 Bullets

  • Diabetes = chronic hyperglycemia from not enough insulin, insulin not working, or both.
  • Insulin (Ξ²-cells) lowers glucose. Glucagon, epinephrine, cortisol, growth hormone all raise it (“counterregulatory”).
  • Type 1 = autoimmune Ξ²-cell destruction β†’ no insulin β†’ ketosis-prone, lean, young, antibody-positive.
  • Type 2 = insulin resistance + relative deficiency β†’ ~90% of all diabetes; obesity, age > 40.
  • HbA1c = glucose stuck on hemoglobin’s Ξ²-chain valine; reflects ~3 months because RBCs live ~120 days.
  • OGTT = drink 75 g glucose, draw blood at fasting and 2 h. Abnormal if 2-h β‰₯ 200 mg/dL.
  • Ketones = Ξ²-hydroxybutyrate (most), acetoacetate, acetone. Nitroprusside misses Ξ²-OH-butyrate (the most abundant in DKA).
  • Albuminuria = the earliest sign of diabetic kidney damage β€” screen with first-morning ACR.
ANALOGY β€” HbA1c is glucose’s “snowfall on a roof”
Imagine each red blood cell is a roof that lasts ~120 days. Every time blood sugar is high, glucose “snows” onto the roof and freezes there permanently. We can’t measure the snowfall day by day β€” but if we sample many roofs at once, the average snow depth tells us how much it snowed over the past 3 months. That’s HbA1c.

Why this matters clinically: a single fasting glucose tells you about right now; HbA1c tells you about the last 8–12 weeks of compliance and control.

Concept β†’ Plain English

Concept Plain-English Explanation
Hyperglycemia Blood sugar too high. Cells aren’t pulling it in or the liver keeps making more.
Insulin resistance The “key” still works but the “lock” is rusted. Cells need way more insulin to take in glucose.
C-peptide The piece “snipped off” when proinsulin β†’ insulin. Made in equal amounts to your own insulin β†’ tells us if YOUR pancreas is working.
Glycation Sugar passively sticks to proteins. No enzyme needed. The longer sugar is high, the more sticking.
Renal threshold The blood glucose level (~180 mg/dL) above which kidneys spill sugar into urine.
Ketone bodies Liver’s “Plan B” fuel made from fat when insulin is low. Acidic β€” blood pH falls.

Section 2 β€” Core Analytes & Methods

Glucose

Feature Details
Definition Primary blood sugar; main fuel for the brain and RBCs.
Reference range Fasting plasma glucose: 70–99 mg/dL (3.9–5.5 mmol/L). Whole blood is ~10–15% lower than plasma.
Mechanism of elevation ↓ insulin secretion (Type 1) or ↓ insulin action (Type 2) β†’ ↓ glucose uptake into muscle/fat (GLUT4) + ↑ hepatic gluconeogenesis & glycogenolysis.
Method Glucose hexokinase (reference) or glucose oxidase. Hexokinase is more specific β€” fewer interferences.
POC meters Glucose oxidase or glucose dehydrogenase on test strips; reflectance photometry or amperometric (electrochemistry). NOT for diagnosis.
Critical values Hypoglycemia < 40–50 mg/dL; severe hyperglycemia > 500 mg/dL or DKA threshold ~250+ mg/dL.
Sample Venous plasma preferred for diagnosis. Capillary whole blood acceptable for SMBG.

Hexokinase is the reference method because it tolerates hemolysis and most drugs. Glucose oxidase strips can be falsely lowered by ascorbic acid and falsely affected by oxygen tension β€” a real issue in arterial vs venous samples on POC meters.

PRE-ANALYTICAL β€” Glucose (the most-tested pre-analytical concept in chemistry)
Glycolysis lowers glucose ~5–7% per hour at room temperature in untreated whole blood (RBCs and WBCs keep eating sugar after the draw).
Stop glycolysis by: (1) plasma separation within 30 min, OR (2) place tube on ice slurry, OR (3) use a sodium fluoride tube (gray top β€” fluoride inhibits enolase). Caveat: fluoride takes ~1–4 hours to fully stop glycolysis. Citrate buffer tubes inhibit glycolysis immediately and are now preferred.
Specimen: venous plasma for diagnosis. Capillary whole blood is for self-monitoring only β€” meters are calibrated to “plasma equivalent” using a 1.11Γ— factor.
Storage: Plasma stable refrigerated 8 h; serum must be separated from cells.

Hemoglobin A1c (HbA1c)

Feature Details
Definition HbA with glucose attached to the N-terminal valine of the Ξ²-chain via a stable Amadori ketoamine.
What it measures Average glucose over the prior 8–12 weeks (RBC lifespan ~120 days). The most recent month contributes ~50%.
Diagnostic cutoff β‰₯ 6.5% (48 mmol/mol) on an NGSP-certified, DCCT-traceable assay.
Pre-diabetes 5.7–6.4% (39–46 mmol/mol).
Treatment target < 7% (53 mmol/mol) for most adults; individualized.
Methods Ion-exchange HPLC (separates by charge), boronate affinity (binds cis-diol of glucose), immunoassay (antibody to glycated Ξ²-chain), capillary electrophoresis, enzymatic.
Method comparison HPLC = excellent precision but Hb variants can co-elute and falsely raise/lower result. Affinity = measures total GHb, less affected by variants. Immunoassay = also disturbed by some variants.
Estimated avg glucose (eAG) eAG (mg/dL) = 28.7 Γ— HbA1c βˆ’ 46.7. (HbA1c 7% β‰ˆ 154 mg/dL average glucose.)

Each 1% rise in HbA1c β‰ˆ 30 mg/dL rise in average glucose. That’s the cleanest “do the math” question on the boards.

PRE-ANALYTICAL β€” HbA1c
Patient does NOT need to fast. EDTA whole blood is standard. Stable at 4Β°C for ~1 week, βˆ’70Β°C for years. Avoid storage at βˆ’20Β°C with ion-exchange methods. Heparin tubes must be assayed within 2 days.
Falsely LOW HbA1c: hemolytic anemia, recent transfusion, acute blood loss, sickle cell trait/disease (depending on method), pregnancy (high RBC turnover), erythropoietin therapy.
Falsely HIGH HbA1c: iron-deficiency anemia, splenectomy, B12/folate deficiency (older RBCs), uremia (carbamylated Hb β€” a problem with older HPLC, not modern methods), chronic alcohol/aspirin (acetylated Hb on older methods).
HbF > 15% β†’ suspect a hemoglobinopathy and choose a method that reports HbA1c accurately (check NGSP.org).

Insulin, C-peptide, Proinsulin

Analyte What it tells you Half-life Liver clearance Best use
Insulin Functional Ξ²-cell output (but ~50% extracted on first liver pass) 4–5 min ~50% first pass Hypoglycemia work-up; insulin resistance research
C-peptide Endogenous insulin secretion (1:1 molar with insulin, NOT cleared by liver) ~35 min None (kidney only) Distinguish endogenous vs exogenous insulin in hypoglycemia; classify Type 1 vs Type 2; insulin pump eligibility
Proinsulin Storage form (low activity); high in Ξ²-cell tumors with defective processing ~30 min ~25% of insulin’s Insulinoma; familial hyperproinsulinemia
KEY POINT β€” The “surreptitious insulin” classic
Patient with hypoglycemia. High insulin + LOW C-peptide = exogenous insulin (factitious / overdose). High insulin + HIGH C-peptide = insulinoma or sulfonylurea (sulfonylureas drive endogenous secretion β†’ C-peptide rises with insulin). The only way to distinguish insulinoma from sulfonylurea abuse is a sulfonylurea screen.

Islet Autoantibodies (Type 1 markers)

Antibody Target % positive in new-onset T1D Note
ICA (Islet Cell Antibody) Cytoplasmic islet antigens 75–85% Immunofluorescence; cumbersome; expressed in JDF units
IAA (Insulin autoantibody) Insulin >90% in children < 5 yr; < 40% if onset after age 12 Develops in ALL patients on exogenous insulin β†’ only useful BEFORE treatment
GAD65 Glutamic acid decarboxylase 65 kDa isoform ~60% Detectable up to 10 yrs before onset; identifies LADA in adults
IA-2A Tyrosine phosphatase IA-2 >50% Often combined with GAD65 in dual assay
ZnT8 Zinc transporter 8 60–80% Picks up ~26% of patients negative for the others
EXAM TIP β€” IAA timing trap
Test IAA before starting insulin therapy. Once exogenous insulin begins, all patients develop antibodies and the result is meaningless.

Ketone Bodies

Ketone % in DKA Detection Notes
Ξ²-Hydroxybutyrate (Ξ²-OHB) ~78% (ratio up to 6:1 over acetoacetate) Enzymatic (Ξ²-OHB dehydrogenase / NAD+ β†’ NADH); some POC meters NOT detected by nitroprusside
Acetoacetate ~20% Nitroprusside (Acetest, Ketostix) The “purple” reaction
Acetone ~2% Nitroprusside (lower sensitivity than acetoacetate) Volatile, “fruity breath”
PRE-ANALYTICAL β€” Ketones
Nitroprusside-based dipsticks (Ketostix, Acetest) detect acetoacetate only. In DKA the ratio of Ξ²-OHB to acetoacetate may be 6:1 β†’ urine ketones can be only weakly positive on presentation. As the patient is treated, Ξ²-OHB converts back to acetoacetate, and dipstick ketones APPEAR to worsen even though the patient is improving. Always quantitate Ξ²-OHB in serum to monitor DKA. False positives: drugs with sulfhydryl groups (captopril). False negatives: very acidic urine, expired strips, large vitamin C dose.

Fructosamine & Glycated Albumin

Marker Window When to use Limitation
Fructosamine 2–3 weeks (albumin tΒ½ ~14–20 d) Hemolytic anemia, hemoglobinopathy, GDM, recent change in therapy Invalid if albumin < 30 g/L; affected by nephrotic syndrome, cirrhosis
Glycated albumin ~2–3 weeks Same indications as fructosamine Altered by thyroid disease, smoking, hyperuricemia
1,5-Anhydroglucitol (1,5-AG) 1–2 weeks (postprandial) Detects postprandial spikes that HbA1c misses Falsely altered by diet, renal disease

Albuminuria (Diabetic Nephropathy Screening)

Category Β΅g/min mg/24 h ACR (mg/g creatinine)
Normal < 20 < 30 < 30
Moderately ↑ (formerly “microalbuminuria”) 20–200 30–300 30–300
Severely ↑ (formerly “macroalbuminuria” / overt) > 200 > 300 > 300
PRE-ANALYTICAL β€” Albuminuria
First morning sample is best (lower within-person variation). Confirm a positive screen by repeating 2 of 3 samples positive within 3–6 months. Avoid sampling after exercise, fever, UTI, marked hyperglycemia, severe hypertension, CHF, or in pregnancy. Quantitative reference method = immunoturbidimetry on a urine sample. Always pair with creatinine and report as ACR to correct for urine concentration.

Section 3 β€” Visual Memory Hooks

🧠 MNEMONIC β€” “126 / 200 / 6.5 / 200” β€” The Four Diagnostic Numbers
1-2-6 β†’ 2-0-0 β†’ 6-point-5 β†’ 2-0-0 with symptoms.
Read it out loud once: “FPG 126, OGTT 200, A1c 6.5, Random 200.” Repeat the same test on a different day to confirm β€” UNLESS unequivocal hyperglycemia.
🧠 MNEMONIC β€” “GICe Cools Glucose”
To stop glycolysis in a glucose tube: Gray-top, Ice slurry, Citrate buffer, or early plasma separation. (“GICe Cools Glucose.”)
🧠 MNEMONIC β€” “GAIA Z” β€” The Type 1 Antibodies
GAD65, Anti-IA-2, Insulin (IAA), Anti-ICA, plus ZnT8. (GAIA Z = the goddess of T1D antibodies.)
🧠 MNEMONIC β€” “BAD” β€” The Three Ketones in DKA
Beta-hydroxybutyrate (most), Acetoacetate, acetone (the volatile Decay product). Nitroprusside misses the B β€” and the B is the BIGGEST.
🧠 MNEMONIC β€” “GECG” Counterregulatory Hormones
Glucagon (immediate, biggest player), Epinephrine (key in T1D when glucagon fails), Cortisol (3–4 h, sustained), Growth hormone (slow, sustained).

Pattern Triggers β€” “If you see X β†’ think Y β†’ because Z”

If you see… Think… Because…
Falsely low glucose, normal everything else Glycolysis in transit (no fluoride / ice / late separation) RBCs metabolize glucose ~5–7%/h post-collection
HbA1c that doesn’t match SMBG average Hemoglobinopathy or altered RBC lifespan HbA1c assumes RBC lifespan = 120 d; hemolysis or transfusion break that assumption
Hypoglycemia with high insulin and low C-peptide Exogenous insulin (factitious or accidental) Pharmaceutical insulin has no C-peptide
Hypoglycemia with high insulin and high C-peptide Insulinoma OR sulfonylurea Both drive endogenous secretion
Negative urine ketones in suspected DKA Ξ²-OHB-dominant ketosis Nitroprusside detects acetoacetate, not Ξ²-OHB
Worsening urine ketones during DKA treatment Ξ²-OHB β†’ acetoacetate conversion Don’t be fooled β€” patient is improving; trust pH/anion gap/serum Ξ²-OHB
HbA1c 5.7–6.4% Pre-diabetes Increased risk of progression and CV disease
FPG 100–125 mg/dL Impaired Fasting Glucose (IFG) Pre-diabetes by ADA criteria
Pregnant woman, fasting 92 mg/dL at 26 weeks GDM by IADPSG one-step criteria Single elevated value diagnostic on 75-g OGTT
HbA1c apparently 14% or 15% Hemoglobin variant interfering with method Re-test by alternate method (affinity vs HPLC vs immuno)

Side-by-Side β€” Type 1 vs Type 2

Feature Type 1 Type 2
% of cases 5–10% ~90%
Mechanism Autoimmune Ξ²-cell destruction (insulinopenia) Insulin resistance + relative deficiency
Onset Abrupt, young (75% < 18 yrs) Insidious, > 40 yr usually
Body habitus Lean Often obese
Ketosis Prone (DKA) Resistant (HHS more common)
C-peptide Low / undetectable Normal or high (early); low (late)
Autoantibodies Positive (GAD65, IA-2, IAA, ICA, ZnT8) Negative
Insulin requirement Mandatory Variable
HLA association Strong (DR3/DR4) Weak

Diagnostic Window β€” Time-Based Markers

Marker Reflects glucose over… Onset of change after Ξ” glucose Best clinical use
Plasma glucose Right now Minutes Acute diagnosis, SMBG, DKA monitoring
1,5-Anhydroglucitol 2–14 days Days Postprandial glycemic excursions
Fructosamine / glycated albumin 2–3 weeks 1–2 weeks GDM, hemolytic anemia, hemoglobinopathy
HbA1c 8–12 weeks (most-recent month β‰ˆ 50%) Weeks Diagnosis, long-term monitoring, microvascular risk prediction

Section 4 β€” Diagnostic Flowcharts

Flowchart 1 β€” A 47-year-old with random glucose 220 mg/dL: do they have diabetes?

Patient with classic symptoms (polyuria, polydipsia, weight loss)β†’ measure casual (random) plasma glucose
β†’ If β‰₯ 200 mg/dL AND symptoms present β†’ Diabetes confirmed β€” no repeat test needed
β†’ If asymptomatic OR < 200 mg/dL β†’ proceed to FPG and/or HbA1c
β†’ FPG β‰₯ 126 mg/dL OR HbA1c β‰₯ 6.5% OR 2-h OGTT β‰₯ 200 mg/dL β†’ repeat the same test on a different day β†’ Diabetes confirmed
⚠️ Common mistake: mixing two different tests for diagnosis (e.g., FPG one day, HbA1c next). ADA says repeat the SAME test on a different day. Mixing methods invalidates the diagnosis.

Flowchart 2 β€” Adult presenting with hypoglycemia: differential

Documented hypoglycemia (Whipple’s triad) β†’ draw insulin + C-peptide + proinsulin + sulfonylurea screen during the event
β†’ Insulin HIGH + C-peptide LOW β†’ Exogenous insulin (factitious / accidental overdose)
β†’ Insulin HIGH + C-peptide HIGH + sulfonylurea NEG β†’ Insulinoma (Ξ²-cell tumor)
β†’ Insulin HIGH + C-peptide HIGH + sulfonylurea POS β†’ Sulfonylurea ingestion
β†’ Insulin LOW + C-peptide LOW + IGF-2 HIGH β†’ Non-islet cell tumor hypoglycemia (IGF-2 mediated)
⚠️ Common mistake: forgetting the sulfonylurea screen β€” it produces an “insulinoma-like” pattern (high insulin + high C-peptide).

Flowchart 3 β€” Pregnant woman at 26 weeks gestation: GDM screening

All pregnant women, 24–28 weeks β†’ choose strategy
One-step (IADPSG): Fasting β†’ 75 g glucose β†’ measure 0, 1, 2 h. β†’ Diagnose GDM if any one exceeds: FPG β‰₯ 92, 1-h β‰₯ 180, 2-h β‰₯ 153 mg/dL.
Two-step (ACOG): Step 1: 50 g non-fasting β†’ 1-h β‰₯ 140 mg/dL β†’ proceed. Step 2: 100 g 3-h OGTT β†’ diagnose GDM if any two exceeded: FPG β‰₯ 95, 1-h β‰₯ 180, 2-h β‰₯ 155, 3-h β‰₯ 140 mg/dL (Carpenter/Coustan).
β†’ Postpartum 6–12 weeks: re-screen with non-pregnant OGTT criteria (NOT HbA1c β€” recent therapy distorts it).
⚠️ Common mistake: using HbA1c to screen for GDM. HbA1c is NOT recommended for GDM diagnosis or postpartum screening within ~3 months due to changing RBC dynamics in pregnancy.

Flowchart 4 β€” HbA1c discordant with SMBG: which method to choose?

HbA1c result doesn’t match patient’s average glucose β†’ rule out altered RBC lifespan first (CBC, retic count, transfusion history, recent blood loss).
β†’ If hemoglobinopathy suspected (HbS, HbC, HbE, HbF > 15%) β†’ check NGSP.org for a method that handles that variant. Boronate affinity measures total GHb and is least affected by charge variants.
β†’ If RBC lifespan is short (hemolysis, transfusion) β†’ abandon HbA1c β†’ Use fructosamine or glycated albumin (~2–3 wk window)
β†’ If iron deficiency / B12 deficiency (older RBCs) β†’ HbA1c may be falsely high β†’ treat the deficiency, then recheck.
⚠️ Common mistake: trusting an HbA1c of 14% from one method without confirming with a second method. Hb variants can produce wildly false values depending on assay chemistry β€” and a single variant can falsely INCREASE in one method and DECREASE in another.

Flowchart 5 β€” Type 1 vs Type 2 in an ambiguous adult patient

Adult presenting with new diabetes β€” typed unclear (lean, possibly autoimmune)
β†’ Measure fasting C-peptide + GAD65 antibody
β†’ C-peptide LOW + GAD65 POSITIVE β†’ Type 1 (or LADA β€” latent autoimmune diabetes of adults)
β†’ C-peptide NORMAL/HIGH + GAD65 NEGATIVE β†’ Type 2 diabetes
β†’ C-peptide LOW + ALL antibodies NEGATIVE β†’ Idiopathic Type 1 (Type 1B) or MODY (genetic testing if young, lean, family hx)
⚠️ Common mistake: testing IAA in a patient already on insulin β€” insulin therapy itself induces the antibody, so the result is not interpretable.

Section 5 β€” 25 High-Yield Facts

# Fact
1 Diabetes is diagnosed by FPG β‰₯ 126, 2-h OGTT β‰₯ 200, HbA1c β‰₯ 6.5%, or random glucose β‰₯ 200 with classic symptoms.
2 Confirm a positive screening result by repeating the SAME test on a different day β€” unless symptoms are unequivocal.
3 Pre-diabetes: FPG 100–125, 2-h OGTT 140–199, or HbA1c 5.7–6.4%.
4 Whole blood glucose is 10–15% lower than plasma glucose; meters are calibrated to report plasma-equivalent values.
5 Glycolysis lowers glucose 5–7% per hour at room temperature; prevent it with ice slurry, sodium fluoride, or citrate buffer.
6 Hexokinase is the reference glucose method; glucose oxidase strips can be lowered by ascorbic acid and altered by oxygen tension.
7 HbA1c reflects 8–12 weeks of glycemic control; the most recent month contributes ~50%.
8 HbA1c forms from the non-enzymatic attachment of glucose to the N-terminal valine of the Ξ²-chain (Amadori ketoamine).
9 Each 1% rise in HbA1c β‰ˆ 30 mg/dL rise in average glucose. eAG (mg/dL) = 28.7 Γ— HbA1c βˆ’ 46.7.
10 HbA1c assays must be NGSP-certified and DCCT-traceable; POC HbA1c is NOT for diagnosis.
11 HbA1c is falsely LOW with hemolysis, recent transfusion, blood loss, or pregnancy (rapid RBC turnover).
12 HbA1c is falsely HIGH with iron deficiency, splenectomy, B12/folate deficiency, or uremia (older HPLC methods).
13 C-peptide measures endogenous insulin; not extracted by liver, longer half-life (~35 min) than insulin (~5 min).
14 Hypoglycemia + high insulin + LOW C-peptide β†’ exogenous insulin. High insulin + HIGH C-peptide β†’ insulinoma or sulfonylurea.
15 Type 1 diabetes antibodies: GAD65, IA-2, IAA, ICA, ZnT8. Positive in 60–90% of new-onset T1D.
16 Order IAA before starting insulin therapy; once insulin is given, IAA becomes universally positive and uninformative.
17 Ξ²-Hydroxybutyrate is the dominant ketone in DKA (ratio up to 6:1 over acetoacetate); nitroprusside-based dipsticks miss it.
18 During DKA treatment, Ξ²-OHB converts to acetoacetate β€” urine ketone tests can appear to worsen even as the patient improves.
19 OGTT for non-pregnant adults uses 75 g glucose; preceded by 3 days of β‰₯ 150 g/day carbohydrate diet and 10–16 h fast.
20 GDM (one-step IADPSG): 75 g OGTT β€” diagnostic if FPG β‰₯ 92, 1-h β‰₯ 180, OR 2-h β‰₯ 153 mg/dL (any one).
21 GDM (two-step Carpenter/Coustan): 50 g screen β†’ if 1-h β‰₯ 140, do 100 g 3-h OGTT β†’ diagnostic if 2 of: FPG β‰₯ 95, 1-h β‰₯ 180, 2-h β‰₯ 155, 3-h β‰₯ 140.
22 Albuminuria screening: first-morning ACR; confirm with 2 of 3 positive samples in 3–6 months. Reference method = immunoturbidimetry.
23 Albuminuria categories: normal < 30, moderately ↑ 30–300, severely ↑ > 300 mg/g creatinine.
24 DCCT (Type 1) and UKPDS (Type 2) showed each 1% reduction in HbA1c reduces microvascular complications by ~25–37%.
25 Counterregulatory hormones (raise glucose): glucagon (immediate, biggest), epinephrine (key when glucagon fails), cortisol (3–4 h, sustained), GH (sustained).
⚠️ EXAM TIP β€” The single most testable pattern in this chapter
Discordance between HbA1c and capillary glucose monitoring β†’ suspect altered RBC lifespan or hemoglobinopathy. The exam writer wants you to NOT change the patient’s insulin dose based on a misleading HbA1c. Switch to fructosamine or glycated albumin until you sort it out.

Quick Interpretation Table

Lab Finding Clinical Interpretation
FPG 126, HbA1c 7.2% on two separate days Diabetes mellitus confirmed
HbA1c 5.9%, FPG 110 Pre-diabetes (IFG + intermediate HbA1c)
HbA1c 14.2%, fingerstick avg ~120 mg/dL Hemoglobin variant interfering β€” re-test by alternate method
HbA1c 5.4%, fingerstick avg ~220 mg/dL Hemolytic anemia or recent transfusion (falsely low HbA1c)
Glucose 200 mg/dL, urine ketones negative, anion gap 24 DKA β€” Ξ²-OHB-dominant; quantitate Ξ²-OHB serum
Hypoglycemia, insulin 50 Β΅U/mL, C-peptide < 0.6 ng/mL Exogenous insulin (factitious or overdose)
Pregnant, 24 wks, FPG 95, 1-h 185 on 75 g OGTT GDM by IADPSG (any one elevated value)
ACR 75 mg/g, repeated Γ— 2 over 3 months Diabetic nephropathy β€” start ACE inhibitor or ARB

🎴 Flashcards β€” Active Recall Practice

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Section 6 β€” Practice Questions (Patient Scenarios)

Practice Quiz β€” Test Your Knowledge

Click an answer for each question. You’ll get instant feedback with a detailed explanation. Your final score appears at the end. Good luck!
0 of 10 answered
Question 1 of 10
A 52-year-old asymptomatic man’s blood is drawn at 8 a.m. after an overnight fast. The plasma is separated 4 hours later because the courier was late. Glucose results 98 mg/dL. The clinician is surprised because three home glucose meter readings last week averaged 145 mg/dL. The most likely explanation is:
Question 2 of 10
A 28-year-old African-American woman has Type 2 diabetes. Her self-monitored glucose averages 180 mg/dL, but her HbA1c result by HPLC is 4.9%. The most likely cause is:
Question 3 of 10
A 42-year-old nurse is brought to the ED with seizures. Glucose: 28 mg/dL. Insulin: 48 Β΅U/mL (high). C-peptide: 0.3 ng/mL (low). Sulfonylurea screen: negative. The most likely diagnosis is:
Question 4 of 10
A 22-year-old Type 1 diabetic presents with DKA. Initial urine dipstick shows trace ketones; 4 hours into IV insulin therapy, urine ketones now read large. Glucose has fallen from 480 to 220 mg/dL. The most likely explanation is:
Question 5 of 10
A 26-year-old at 26 weeks of gestation undergoes a 75-g 2-h OGTT. Results: FPG 95, 1-h 165, 2-h 156 mg/dL. Using the IADPSG one-step criteria, what is the diagnosis?
Question 6 of 10
A 48-year-old man with newly diagnosed diabetes has FPG 142 mg/dL and HbA1c 6.9%. His ACR is 56 mg/g (random urine). The next best step is:
Question 7 of 10
A laboratory is implementing a new glucose tube. Which of the following will most rapidly halt glycolysis in whole blood?
Question 8 of 10
A 35-year-old woman with newly diagnosed adult-onset diabetes is lean, has a strong family history of T1D, and a fasting C-peptide of 0.4 ng/mL. Which test is most appropriate to clarify the diagnosis?
Question 9 of 10
A patient with sickle cell disease (HbS) needs glycemic monitoring. Their HbA1c by HPLC has fluctuated wildly between 4% and 14% on consecutive visits. Which marker is the most reliable alternative?
Question 10 of 10
Which of the following is the MOST appropriate goal HbA1c for an otherwise healthy 50-year-old patient with Type 2 diabetes (no significant comorbidities)?

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Section 7 β€” Trap Questions (T1–T7)

Trap Questions β€” Avoid the Pitfalls

These are the most commonly missed exam questions. Click an answer to see the trap explained and learn how to avoid the mistake.
0 of 7 answered
Question 1 of 7
A 58-year-old man has FPG 132 mg/dL on Monday and an HbA1c of 6.7% on Friday. Both are above diagnostic cutoffs. Is he diabetic?
Question 2 of 7
An iron-deficient woman has HbA1c of 8.4%. After 3 months of iron repletion (HbA1c assay unchanged) the new value is 6.8%. Her glucose monitoring shows no significant change in average glucose. The 8.4% was most likely:
Question 3 of 7
A capillary glucose meter reads 30 mg/dL on a critically ill ICU patient with hematocrit 18%. The patient is alert. The most likely cause is:
Question 4 of 7
Urine ketones in a stable Type 1 diabetic on insulin: trace positive throughout pregnancy. Glucose averages 130 mg/dL. Is the patient in DKA?
Question 5 of 7
A homeless patient is found unconscious. Glucose 38 mg/dL. After dextrose, he wakes up. Insulin level: 12 Β΅U/mL (normal). C-peptide: 1.6 ng/mL (normal). Sulfonylurea screen: positive for glipizide. The most likely diagnosis is:
Question 6 of 7
A pediatric patient with fasting glucose 145, 2-h OGTT 240 mg/dL, no detectable autoantibodies (GAD65, IA-2, IAA, ZnT8 all negative), normal C-peptide, and a strong autosomal dominant family history of diabetes by age 25. The most likely diagnosis is:
Question 7 of 7
A 65-year-old patient on dialysis (BUN 80, creatinine 6.5) has an HbA1c of 9.5% by HPLC. The patient’s average glucose by SMBG is 130 mg/dL. The discordance is most likely due to:

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Section 8 β€” Rapid Revision Sheet

πŸ“– READ THIS 2 HOURS BEFORE YOUR EXAM

Diagnostic Cutoffs

  • FPG β‰₯ 126 mg/dL
  • 2-h OGTT β‰₯ 200 mg/dL
  • HbA1c β‰₯ 6.5%
  • Random β‰₯ 200 + symptoms
  • Pre-DM: FPG 100–125, A1c 5.7–6.4

GDM (75 g IADPSG)

  • FPG β‰₯ 92
  • 1-h β‰₯ 180
  • 2-h β‰₯ 153
  • Any 1 = GDM

GDM (100 g Carpenter/Coustan, 2 of 4)

  • FPG β‰₯ 95
  • 1-h β‰₯ 180
  • 2-h β‰₯ 155
  • 3-h β‰₯ 140

HbA1c Math

  • 1% Ξ” β‰ˆ 30 mg/dL Ξ” avg glucose
  • Target < 7% (most adults)
  • Window: 8–12 weeks
  • Last month β‰ˆ 50% of value

Albuminuria

  • Normal < 30 mg/g
  • Mod ↑ 30–300 mg/g
  • Severe ↑ > 300 mg/g
  • Confirm 2 of 3 in 3–6 mo

Reference Ranges

  • FPG normal: 70–99 mg/dL
  • HbA1c: 4–6%
  • Insulin (fasting): 12–150 pmol/L (2–25 Β΅IU/mL)
  • C-peptide (fasting): 0.78–1.89 ng/mL

Pattern Associations

  • HbA1c β†’ Ξ²-chain valine glycation
  • Hexokinase β†’ reference glucose
  • Boronate affinity β†’ measures total GHb (Hb-variant resistant)
  • HPLC β†’ separates HbA1c by charge

Hormones

  • Insulin ↓ glucose
  • Glucagon ↑ first (minutes)
  • Epinephrine ↑ when glucagon fails (T1D)
  • Cortisol/GH ↑ over 3–4 hours

Antibody Pairings

  • GAD65 + IA-2 = best dual screen
  • IAA β†’ only valid before insulin
  • ZnT8 β†’ catches 26% missed by others
  • ICA β†’ expressed in JDF units

HbA1c Falsely HIGH vs LOW

  • HIGH: iron deficiency, splenectomy, B12/folate def, uremia (older HPLC)
  • LOW: hemolysis, transfusion, blood loss, pregnancy, EPO

Insulin / C-peptide

  • Insulinoma: ↑ insulin, ↑ C-peptide
  • Sulfonylurea: ↑ insulin, ↑ C-peptide (drug screen +)
  • Exogenous insulin: ↑ insulin, ↓ C-peptide
  • IGF-2 tumor: ↓ insulin, ↓ C-peptide

Glucose Meters & Hematocrit

  • Low Hct β†’ meter reads HIGH
  • High Hct β†’ meter reads LOW
  • Inaccurate < 60 or > 500 mg/dL
  • Capillary > venous plasma by 1.11Γ—

Type 1 vs Type 2

  • T1: lean, young, autoab+, low C-peptide, ketosis
  • T2: obese, >40, autoabβˆ’, normal/high C-peptide, HHS
  • LADA = adult T1 with positive GAD65

Ketones β€” the BAD trap

  • Ξ²-OHB (most) β€” NOT detected by nitroprusside
  • Acetoacetate β€” IS detected
  • Acetone β€” minor, “fruity breath”
  • Tx β†’ Ξ²-OHB β†’ acetoacetate (dipstick “worsens”)

Glucose Tubes

  • Sodium fluoride (gray) β€” slow (1–4 h)
  • Citrate buffer β€” immediate
  • Ice slurry β€” universal hack
  • Plasma separation < 30 min β€” best

Glycated Markers β€” Window

  • SMBG: now
  • 1,5-AG: 2–14 days (postprandial)
  • Fructosamine / glycated albumin: 2–3 weeks
  • HbA1c: 8–12 weeks

Common Pre-DM Confusions

  • IFG (FPG 100–125) β‰  IGT (2-h 140–199)
  • HbA1c 5.7–6.4 β‰  same patients as IFG/IGT
  • Pre-DM = high CV + DM risk regardless

HbA1c Methods

  • HPLC β€” charge β†’ variant interference
  • Boronate affinity β€” cis-diol binding β†’ variant tolerant
  • Immunoassay β€” antibody to glycated Ξ²-chain
  • All must be NGSP / DCCT-traceable

Section 9 β€” Final Memory Map

β–£ Diabetes Mellitus
β—† Classification
β€’ Type 1 (5–10%) β€” autoimmune Ξ²-cell destruction β†’ low C-peptide, antibody+
β€’ Type 2 (~90%) β€” insulin resistance + relative deficiency β†’ obesity, age > 40
β€’ GDM β€” onset/recognition in pregnancy
β€’ Other (MODY, drug-induced, endocrinopathies, exocrine pancreas dz)
β—† Diagnostic Numbers
β€’ FPG β‰₯ 126 mg/dL (after 8 h fast)
β€’ 2-h OGTT β‰₯ 200 mg/dL (75 g)
β€’ HbA1c β‰₯ 6.5% (NGSP-certified)
β€’ Random β‰₯ 200 mg/dL + classic symptoms
β€’ Confirm same test on different day
β—† Glucose Methods & Pre-analytics
β€’ Hexokinase (reference) vs glucose oxidase
β€’ Glycolysis 5–7%/h β†’ ice / NaF / citrate
β€’ Whole blood ~10–15% < plasma; meters calibrated Γ— 1.11 to plasma
β€’ Meters NOT for diagnosis; affected by Hct, drugs, hypoxia
β—† HbA1c
β€’ Glucose on N-terminal valine of Ξ²-chain (Amadori)
β€’ 8–12 wk window; 1% β‰ˆ 30 mg/dL avg
β€’ Methods: HPLC, boronate affinity, immunoassay
β€’ Falsely ↑: iron def, B12 def, splenectomy, uremia
β€’ Falsely ↓: hemolysis, transfusion, pregnancy, EPO
β€’ Hb variants (S, C, E, F > 15%) β†’ method-specific bias
β—† Insulin / C-peptide / Proinsulin
β€’ Insulin tΒ½ 4–5 min; 50% extracted by liver first pass
β€’ C-peptide tΒ½ ~35 min; not extracted by liver β†’ better Ξ²-cell index
β€’ Hypoglycemia: ↑ insulin + ↓ C-peptide = exogenous
β€’ Hypoglycemia: ↑ insulin + ↑ C-peptide = insulinoma OR sulfonylurea
β—† Type 1 Antibodies (GAIA Z)
β€’ GAD65 (~60%, up to 10 yr before)
β€’ IA-2A (>50%); ZnT8 (60–80%); IAA (90% in <5 yo, <40% >12 yo)
β€’ ICA (75–85%); IAA must be drawn BEFORE insulin therapy
β—† Ketone Bodies
β€’ Ξ²-OHB (~78%) β€” NOT detected by nitroprusside
β€’ Acetoacetate (~20%) β€” IS detected
β€’ Acetone (~2%) β€” volatile, “fruity breath”
β€’ Tx of DKA: Ξ²-OHB β†’ acetoacetate β†’ dipstick “worsens”
β—† Glycated Proteins (alternates to HbA1c)
β€’ Fructosamine β€” 2–3 wk window; need albumin β‰₯ 30 g/L
β€’ Glycated albumin β€” 2–3 wk; altered by thyroid, smoking
β€’ 1,5-AG β€” 2–14 day; sensitive to postprandial spikes
β—† GDM (75 g vs 100 g)
β€’ IADPSG one-step: 75 g β†’ FPG 92 / 1h 180 / 2h 153 (any 1)
β€’ Carpenter/Coustan two-step: 50 g screen β†’ 100 g 3 h β†’ FPG 95 / 1h 180 / 2h 155 / 3h 140 (2 of 4)
β€’ Postpartum: re-screen at 6–12 wk with non-pregnant OGTT (NOT HbA1c)
β—† Albuminuria
β€’ First-morning ACR; 2 of 3 positives in 3–6 mo
β€’ Mod ↑ 30–300 mg/g; severe ↑ > 300 mg/g
β€’ Treat with ACE-I or ARB
β€’ Reference method = immunoturbidimetry
β—† Counterregulatory Hormones (GECG)
β€’ Glucagon β€” Ξ±-cells, immediate, glycogenolysis
β€’ Epinephrine β€” adrenal medulla, key when glucagon fails
β€’ Cortisol β€” sustained gluconeogenesis
β€’ GH β€” chronic, antagonizes insulin
β—† Treatment Targets
β€’ HbA1c < 7% (most adults; ADA)
β€’ HbA1c < 6.5% (some organizations)
β€’ DCCT/UKPDS: each 1% ↓ HbA1c β†’ 25–37% ↓ microvascular risk

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