Carbohydrates Part 1 — Metabolism & Glucose

Clinical Chemistry — ASCP Exam Prep Carbohydrates Part 1 — Metabolism & Glucose

📘 Chapter — Carbohydrates
ASCP Board Exam — Lab-Focused Study Guide
Methods • Clinical Interpretation • Pre-analytical Variables • Practice Questions

Section 0 — Priority Reference Table

Before studying, know what matters. The ASCP MLS exam is NOT a biochemistry exam. It tests whether you can work in a clinical laboratory, choose the right method, recognize interferences, and interpret results.

Priority	Topic	Why It Matters	Exam Focus
HIGH	Glucose Measurement Methods	Most tested lab section	Glucose oxidase vs hexokinase — interferences
HIGH	HbA1c Methods & Interferences	Frequent scenario questions	Affinity vs ion-exchange; hemoglobinopathy effects
HIGH	Diabetes Diagnosis Criteria	ADA values must be memorized	FPG ≥126, HbA1c ≥6.5%, 2-hr OGTT ≥200
HIGH	Type 1 vs Type 2 Diabetes	Classic comparison questions	Ketoacidosis vs hyperosmolar coma
HIGH	Pre-analytical: Glucose Specimens	High yield specimen handling	Gray top tube; separate within 30 min; fluoride mechanism
HIGH	Ketone Bodies — Methods	Nitroprusside trap questions	Detects acetoacetate ONLY — not β-hydroxybutyrate
HIGH	Microalbuminuria	Diabetic complication monitoring	ACR 30–300 µg/mg; preferred = spot collection
MEDIUM	Glycogen Storage Diseases	Classic scenario: von Gierke	Type I = G6Pase deficiency; hypoglycemia + lactic acidosis
MEDIUM	Hypoglycemia Classification	Whipple’s triad	Insulinoma criteria; C-peptide interpretation
MEDIUM	Hormonal Regulation	Mechanism questions	Insulin lowers; glucagon, epinephrine, cortisol raise
MEDIUM	OGTT — Procedure Details	Patient prep questions	3-day carb diet; 75g load; morning test
MEDIUM	Prediabetes Criteria	Screening scenario	IFG 100–125; IGT 140–199 2-hr; HbA1c 5.7–6.4%
LOW	Carbohydrate Chemistry	Background only	Reducing vs nonreducing sugars; sucrose nonreducing
LOW	Galactosemia / Fructosuria	Inborn errors	Reducing substance positive + glucose oxidase negative

Focus on HOW tests work, WHAT results mean clinically, and WHY certain methods have specific interferences. Chemistry structures are LOW priority.

1. Carbohydrate Chemistry (Background)

Feature	Details
Definition	Compounds containing C, H, O; formula Cx(H₂O)y; contain C=O and –OH groups
Classification by carbons	Trioses (3C), Tetroses (4C), Pentoses (5C), Hexoses (6C) — glucose is hexose
Aldose vs Ketose	Aldose: terminal carbonyl (aldehyde) — glucose. Ketose: internal carbonyl (ketone) — fructose
D vs L form	Hydroxyl on highest-numbered asymmetric carbon. D-form = OH to the RIGHT in Fisher projection. Almost all human sugars are D-form
α vs β glucose	α-D: OH at C1 on RIGHT. β-D: OH at C1 on LEFT. Glucose oxidase reacts ONLY with β-D-glucose
Mutarotation	Spontaneous α/β interconversion in solution. Equilibrium: 36% α, 64% β. Allow calibrators 2 hrs to equilibrate before glucose oxidase assay
Haworth projection	Ring form; pyranose = 6-membered ring (5C + 1O); furanose = 5-membered ring

Reducing vs Non-Reducing Sugars

Classification	Sugar Type
REDUCING (free aldehyde/ketone)	Glucose, Fructose, Galactose, Maltose, Lactose
NON-REDUCING	Sucrose (ONLY common nonreducing disaccharide — both anomeric carbons linked)
Clinical relevance	Clinitest/Benedict detects ALL reducing sugars. Glucose oxidase is SPECIFIC for β-D-glucose only

Sugar Units

Type	Definition / Examples
Monosaccharide	Single unit. Examples: glucose, fructose, galactose
Disaccharide	2 monosaccharides. Maltose (Glc+Glc, α-1,4), Lactose (Gal+Glc), Sucrose (Glc+Fru, 1,2 — nonreducing)
Oligosaccharide	2–10 sugar units
Polysaccharide	Starch (plants), Glycogen (animals — liver + muscle). Amylase hydrolyzes starch
Glycogen structure	α-1,4 linkages with α-1,6 branch points every 8–12 residues. More branched than amylopectin

Sucrose is the ONLY major nonreducing disaccharide — won’t react with Benedict/Clinitest. Lactose and maltose ARE reducing sugars. Positive Clinitest + Negative glucose oxidase dipstick = non-glucose reducing substance (think galactosemia in neonates).

2. Glucose Metabolism & Hormonal Regulation

Key Metabolic Pathways

Pathway	Description / Significance
Glycolysis (Embden-Meyerhof)	Glucose → Pyruvate (aerobic) or Lactate (anaerobic). Net: 2 ATP per glucose anaerobically. Pyruvate → Acetyl-CoA → TCA cycle
Gluconeogenesis	Glucose synthesis from NON-carbohydrate sources: amino acids, glycerol, lactate. Liver and kidney
Glycogenesis	Glucose → Glycogen (storage). Glucose-6-P → Glucose-1-P → UDP-glucose → Glycogen. Key enzyme: glycogen synthase
Glycogenolysis	Glycogen → Glucose-6-P (then → glucose in liver only). Key enzyme: glycogen phosphorylase
HMP Shunt	Produces NADPH (protects RBCs from oxidative damage) and Ribose-5-P (DNA/RNA). Important in RBCs
Lipogenesis / Lipolysis	Excess glucose → fatty acids. Fat breakdown → glycerol + fatty acids → ketones when unregulated
Liver vs Muscle glycogen	LIVER has glucose-6-phosphatase → can release glucose to blood. MUSCLE lacks this enzyme → glycogen used only locally for energy

Hormonal Regulation — Master Table

Hormone	Source	Action	Mechanism
Insulin	β-cells, Islets of Langerhans	↓ DECREASES	↑ glucose uptake (muscle/adipose); ↑ glycogenesis, glycolysis, lipogenesis; inhibits glycogenolysis & gluconeogenesis
Glucagon	α-cells, Islets of Langerhans	↑ INCREASES	Stimulates hepatic glycogenolysis & gluconeogenesis; activates lipolysis
Epinephrine	Adrenal medulla	↑ INCREASES	Inhibits insulin; stimulates glycogenolysis & lipolysis. Released during stress
Cortisol	Adrenal cortex (via ACTH)	↑ INCREASES	↓ glucose entry into cells; ↑ gluconeogenesis & liver glycogen; promotes lipolysis
Growth Hormone	Anterior pituitary	↑ INCREASES	↓ glucose entry into cells; ↑ glycolysis. Stimulated by low glucose
ACTH	Anterior pituitary	↑ INCREASES (indirect)	Stimulates cortisol release; converts liver glycogen to glucose
Thyroxine	Thyroid	↑ INCREASES	↑ glycogenolysis, gluconeogenesis, intestinal glucose absorption
Somatostatin	δ-cells, Islets of Langerhans	↑ INCREASES	Inhibits insulin, glucagon, GH — net increase in blood glucose

INSULIN is the ONLY hormone that DECREASES blood glucose. ALL other hormones listed increase blood glucose. This is a classic ASCP trap — they may list multiple hormones and ask which one lowers glucose.

Glucagon is released from α cells. Insulin from β cells. Somatostatin from δ cells. All in the Islets of Langerhans of the pancreas.

3. Hyperglycemia & Diabetes Mellitus

ADA Diagnostic Criteria for Diabetes Mellitus

Test	Diagnostic Threshold (Any ONE = provisional DM)
Fasting Plasma Glucose (FPG)	≥ 126 mg/dL (7.0 mmol/L) — patient fasted ≥ 8 hrs
2-Hour OGTT	≥ 200 mg/dL (11.1 mmol/L) — after 75g glucose load
HbA1c	≥ 6.5% — must use NGSP-certified, DCCT-standardized method
Random Plasma Glucose + Symptoms	≥ 200 mg/dL with classic symptoms (polyuria, polydipsia, weight loss)
CONFIRMATION RULE	Any single criterion (except symptomatic random glucose) must be CONFIRMED by repeat testing on a different day
NOT recommended for diagnosis	Point-of-care (POC) glucose or POC HbA1c

Categories of Fasting Plasma Glucose

Category	FPG Value
Normal	70–99 mg/dL (3.9–5.5 mmol/L)
Impaired Fasting Glucose (IFG / Prediabetes)	100–125 mg/dL
Provisional Diabetes	≥ 126 mg/dL — confirm on second day

Categories of 2-Hour OGTT Glucose

Category	2-Hour PG Value
Normal glucose tolerance	≤ 140 mg/dL
Impaired Glucose Tolerance (IGT / Prediabetes)	140–199 mg/dL
Provisional Diabetes	≥ 200 mg/dL — confirm

Prediabetes — Three Categories

Category	Cutoff
Impaired Fasting Glucose (IFG)	FPG 100–125 mg/dL
Impaired Glucose Tolerance (IGT)	2-hr OGTT 140–199 mg/dL
At-risk HbA1c	5.7%–6.4%

Type 1 vs Type 2 Diabetes vs GDM

Feature	Type 1 DM	Type 2 DM	GDM
Pathogenesis	Autoimmune β-cell destruction → absolute insulin deficiency	Insulin resistance + relative insulin deficiency	Glucose intolerance during pregnancy
Autoantibodies	YES: ICA, IAA, GAD65, IA-2, IA-2B (85–90%)	None	N/A
Insulin level	Very low / absent	Normal or elevated	Variable
Ketosis tendency	HIGH	LOW	LOW
Acute complication	DKA	HHS (Hyperosmolar)	Perinatal complications
Onset	Abrupt; childhood/adolescence	Gradual; adult onset	During pregnancy
Obesity	No	Strong (abdominal)	Pregnancy hormones
Treatment	Insulin required	Lifestyle, oral agents ± insulin	Diet ± insulin

DKA — Type 1

Mechanism	Absolute insulin deficiency + excess glucagon → uncontrolled lipolysis → β-oxidation → acetoacetate + β-hydroxybutyrate + acetone
Glucose	Typically 300–500 mg/dL
Ketones	POSITIVE serum & urine; acidosis (low pH, low bicarbonate)
Electrolytes	Hyponatremia, Hyperkalemia (acidosis shifts K⁺ out — but total body K⁺ is LOW), ↓ HCO₃⁻, anion gap >16 mmol/L
Kussmaul respiration	Deep rapid breathing — blows off CO₂
Fruity breath	Exhaled ACETONE

HHS — Type 2

Who	Elderly Type 2 DM; precipitated by illness, stroke, pancreatitis
Glucose	EXTREMELY high — often >600–1000 mg/dL
Ketones	ABSENT — residual insulin suppresses lipolysis
Dehydration	SEVERE — impairs renal glucose excretion
Bicarbonate	Normal or only slightly decreased
Osmolality	Grossly elevated (>320 mOsm/kg)
BUN / Creatinine	Elevated (dehydration, prerenal azotemia)

DKA = Type 1, ketones POSITIVE, anion gap elevated. HHS = Type 2, ketones ABSENT, glucose >600–1000. The ABSENCE of ketones in HHS is because even small amounts of insulin inhibit lipolysis. Heavily tested.

ADA NUMBERS to memorize: FPG ≥126, 2hr OGTT ≥200, HbA1c ≥6.5%, Random ≥200 + symptoms. Prediabetes: FPG 100–125, 2hr 140–199, HbA1c 5.7–6.4%.

4. Gestational Diabetes Mellitus (GDM)

Screening & Diagnosis

Parameter	Details
When to screen	ALL non-diabetic pregnant women at 24–28 weeks gestation
Test used	2-hour OGTT with 75g glucose load; measure fasting, 1-hour, 2-hour glucose
Fasting glucose	≥ 92 mg/dL (5.1 mmol/L)
1-hour glucose	≥ 180 mg/dL (10.0 mmol/L)
2-hour glucose	≥ 153 mg/dL (8.5 mmol/L)
Diagnosis rule	ANY ONE of the three values = GDM
Overt diabetes in pregnancy	High-risk women diagnosed at FIRST prenatal visit by standard criteria → OVERT diabetes, NOT GDM
Postpartum	Most return to normal; ↑ lifetime risk of Type 2 DM

Neonatal Complications of GDM

Complication	Mechanism
Macrosomia (large baby)	Maternal hyperglycemia → fetal hyperinsulinemia → fetal growth stimulation
Neonatal hypoglycemia	Fetal insulin excess; cord cut → maternal glucose stops abruptly → severe hypoglycemia
Respiratory distress syndrome	Insulin inhibits surfactant synthesis
Hypocalcemia	Disrupted calcium-magnesium balance
Hyperbilirubinemia	Increased RBC breakdown (polycythemia)

GDM criteria are DIFFERENT from regular DM. Fasting ≥92 (not 126), 1-hr ≥180, 2-hr ≥153. Lower thresholds because of fetal risk. Only ONE value needs to be met.

5. Hypoglycemia

Clinical Thresholds

Threshold	Glucose Level
Counterregulatory hormones released	65–70 mg/dL
Symptoms appear	~50–55 mg/dL
CNS dysfunction begins	<50 mg/dL
Severe CNS dysfunction	<20–30 mg/dL
Whipple’s Triad	(1) Symptoms of hypoglycemia, (2) Low plasma glucose, (3) Relief of symptoms after glucose

Causes of Hypoglycemia

Category	Cause	Key Points
DRUGS (most common)	Insulin, sulfonylureas, ethanol, pentamidine, quinine	Sulfonylureas stimulate endogenous insulin + C-peptide (mimics insulinoma; differentiate by drug detection)
Insulinoma	Endogenous insulin-producing β-cell tumor	Low glucose + high insulin. C-peptide ELEVATED. Confirm with 72-hr fast
Factitious (exogenous insulin)	Self-administered insulin	Low glucose + high insulin BUT C-peptide LOW
Hepatic failure	Impaired gluconeogenesis & glycogen storage	Need >80% liver dysfunction
Hormone deficiency	Cortisol, glucagon, epinephrine, GH	More common in children
Neonatal hypoglycemia	Prematurity, maternal DM, GDM, eclampsia	Usually transient; normal neonatal glucose lower than adults
Reactive (postprandial)	1–3 hrs after eating	OGTT NOT recommended for diagnosis
Glycogen storage diseases	Enzyme deficiencies	Von Gierke (Type I): severe fasting hypoglycemia + lactic acidosis

Insulinoma Diagnostic Criteria (72-Hour Fast)

Criterion	Threshold
Plasma glucose at end of fast	< 55 mg/dL with symptoms
Plasma insulin	≥ 18 pmol/L
C-peptide	≥ 0.2 nmol/L (endogenous source)
Proinsulin	≥ 5.0 pmol/L
β-hydroxybutyrate	≤ 2.7 mmol/L (suppressed by excess insulin)
Response to glucagon	Glucose rise ≥ 25 mg/dL

C-PEPTIDE is the key to distinguish insulinoma from factitious (exogenous insulin) hypoglycemia. Insulinoma: HIGH insulin + HIGH C-peptide. Exogenous insulin: HIGH insulin + LOW C-peptide. C-peptide is cleaved from proinsulin only with endogenous production.

6. Glucose Measurement Methods (HIGH PRIORITY)

Glucose Oxidase Method

Feature	Details
Step 1	Glucose Oxidase: β-D-Glucose + O₂ + H₂O → Gluconic acid + H₂O₂
Step 2 (Trinder)	Peroxidase: H₂O₂ + reduced chromogen → oxidized chromogen (color) + H₂O
Specificity	HIGHLY SPECIFIC for β-D-glucose ONLY
Monitoring	(1) Polarographic — measures O₂ consumption (avoids peroxidase interferences); (2) Colorimetric (Trinder)
FALSE DECREASE	Uric acid, bilirubin, ascorbic acid, hemoglobin, glutathione (compete in peroxidase step)
FALSE INCREASE	Strong oxidizing agents (bleach/hypochlorite)
Urine	NOT suitable — urine has high uric acid
CSF	YES — suitable
Mutarotase	Speeds α→β conversion (otherwise 2 hrs needed)

Hexokinase Method (Reference)

Feature	Details
Step 1	Hexokinase: Glucose + ATP → G6P + ADP (Mg²⁺)
Step 2	G6PD: G6P + NADP⁺ → 6-Phosphogluconate + NADPH + H⁺
Measurement	NADPH measured at 340 nm
Status	REFERENCE METHOD
FALSE DECREASE	Gross hemolysis (>0.5 g Hgb/dL — phosphate esters interfere), extreme bilirubin
Unaffected by	Ascorbic acid, uric acid (unlike glucose oxidase)
Suitable specimens	Serum, plasma (heparin, EDTA, fluoride, oxalate, citrate); urine, CSF, serous fluids
Linearity	0–500 mg/dL; dilute with isotonic saline if >500
Lipemia	TG ≥500 mg/dL → positive interference; use sample blank

Glucose Dehydrogenase Method

Feature	Details
Reaction	β-D-Glucose + NAD⁺ → Gluconolactone + NADH (GDH from Bacillus cereus)
Advantage	Highly specific; agrees with hexokinase
CRITICAL FALSE INCREASE	GDH-PQQ variant: maltose, icodextrin (peritoneal dialysis), galactose → falsely elevated POC glucose
GDH-NAD variant	NOT affected by maltose/icodextrin — safer in dialysis patients
Clinical concern	Icodextrin peritoneal dialysis → falsely high POC → insulin overdose → hypoglycemia death risk

Method Comparison Summary

Feature	Glucose Oxidase	Hexokinase	GDH-PQQ
Specificity	β-D-glucose only	Glucose only (highly specific)	Glucose + maltose + icodextrin
Recommended for urine	NO	YES	YES (NAD form)
False LOW	Ascorbic acid, bilirubin, uric acid	Gross hemolysis	None significant
False HIGH	Bleach	Lipemia >500 TG	Maltose, icodextrin, galactose
Reference method	No	YES	No
Measures NADPH at	N/A	340 nm	340 nm

For URINE glucose: use hexokinase or GDH-NAD — NOT glucose oxidase (uric acid in urine falsely lowers results). For BLOOD/SERUM: hexokinase is reference. GDH-PQQ on POC meters is dangerous in patients receiving icodextrin dialysis.

7. Pre-Analytical Variables: Glucose

Specimen Collection — Critical Points

Variable	Details
Preferred specimen	Plasma or serum. VENOUS plasma recommended by ADA
Whole blood vs plasma	Whole blood ~10–12% LOWER than plasma (lower water content)
Capillary vs venous	Fasting: capillary ~2–5 mg/dL HIGHER. Post-load: capillary 20–70 mg/dL HIGHER
Serum vs plasma	Essentially the SAME with modern methods
Gray-top tube (NaF/oxalate)	Sodium fluoride INHIBITS ENOLASE — prevents glycolysis. Also K-oxalate as anticoagulant
Fluoride limitation	Does NOT stop glycolysis immediately — same rate of decline in first 1 hour with or without fluoride
Better alternative	Citrate buffer + NaF + EDTA tubes
If delay expected	Place in ice-water slurry; separate cells within 30 minutes
Rate of glycolysis	~5–7% per hour at room temperature
Leukocytosis risk	↑ WBC → faster glycolysis → up to 65 mg/dL difference in 1–2 hrs
Serum stability	8 hrs at 25°C, 72 hrs at 4°C (after separation)
Fasting requirements	8–10 hour fast (not >16 hr) in the MORNING
Diurnal variation	Afternoon testing may MISS diabetes due to lower afternoon glucose

Specimen rules: Fluoride (gray top) if delay expected; otherwise separate plasma within 30 minutes. Glucose declines 5–7% per hour at room temp. Fluoride inhibits enolase only — does NOT prevent glycolysis in first hour. For 24-hr urine: 5 mL glacial acetic acid (pH 4–5). CSF: analyze immediately. Urine may lose 40% glucose after 24 hrs at room temp. FBG is HIGHER in morning — test in morning.

Reference Intervals for Glucose

Specimen / Population	Fasting Glucose Reference Range
Plasma/Serum — Adults	74–99 mg/dL
Plasma/Serum — Children	60–100 mg/dL
Premature neonates	20–60 mg/dL
Term neonates	30–60 mg/dL
Whole blood	65–95 mg/dL
CSF	40–70 mg/dL (~60% of concurrent plasma)
Urine (24-hr)	1–15 mg/dL
Renal threshold for glucosuria	~180 mg/dL plasma glucose

8. HbA1c / Glycated Hemoglobin (HIGH PRIORITY)

HbA1c Basics

Feature	Details
Mechanism	Glucose reacts NONENZYMATICALLY with the amino group of hemoglobin → ketoamine (Amadori product). Glucose attaches to N-terminal valine of β chains of HbA
Rate of formation	Directly proportional to average plasma glucose
Time window	Average glucose over past 2–3 months (120-day RBC lifespan)
Normal range	4.0%–6.0%
Diagnostic cutoff (DM)	≥ 6.5% (NGSP-certified, DCCT-standardized)
Prediabetes	5.7%–6.4%
Goal (nonpregnant adults)	< 7.0%
Specimen	EDTA whole blood
Standardization	NGSP/DCCT (USA); IFCC reference method (international)
eAG formula	eAG (mg/dL) = 28.7 × HbA1c% − 46.7
Monitoring	Stable at goal: every 6 months. Unstable/not at goal: every 3 months

HbA1c Methods — Charge-Based vs Structure-Based

Method	Principle	Temp Dep?	Hemoglobinopathy Effect
Affinity Chromatography (PREFERRED)	Boronate resin binds glycosylated Hgb via cis-diol groups	NO	NOT affected by Hgb F, S, or C
Cation Exchange (HPLC)	Negatively charged Hgb binds positively charged resin; glyco-Hgb elutes first	YES	Hgb F → false ↑; Hgb S, C → false ↓
Immunoassay (POC)	Antibodies to glycated N-terminal β-chain	Varies	Hgb F >10%: false ↓
Electrophoresis	Separation by charge	Variable	Hgb F >7% interferes
Isoelectric Focusing	By isoelectric point	Variable	Pre-HbA1c interferes
HPLC (IFCC reference)	Separates HbA1a, A1b, A1c	Variable	Reference standard only

Factors Affecting HbA1c (Non-Glucose Factors)

Condition	Effect on HbA1c
Hemolytic anemia / shortened RBC lifespan	FALSE LOW
Iron deficiency / B12/folate deficiency	FALSE HIGH (older RBCs)
Hgb S, C (cation exchange)	FALSE LOW
Hgb F (fetal) — cation exchange	FALSE HIGH
Hgb F — immunoassay (>10%)	FALSE LOW
Pre-HbA1c (aldimine form)	FALSE HIGH in IEF
Uremia / chronic renal failure	FALSE LOW (carbamylation)
Splenomegaly	FALSE LOW
Pregnancy	May not reflect true average

AFFINITY CHROMATOGRAPHY is the PREFERRED clinical method — NOT temperature dependent and NOT affected by Hgb S, C, or F. Ion-exchange (HPLC) is affected by Hgb S and C (false low) and Hgb F (false high). Classic exam distinction.

Two factors determine HbA1c: (1) average plasma glucose over 2–3 months, AND (2) RBC lifespan. If RBC lifespan is decreased (hemolysis, sickle cell), HbA1c will be falsely low even if glucose is high.

9. Oral Glucose Tolerance Test (OGTT)

OGTT Standard Protocol

Parameter	Requirement
Diet preparation	3 days of normal-to-high carbohydrate intake (>150g/day) before test
Fasting	10–16 hours (not <10, not >16)
Timing	Test in the MORNING (diurnal effect)
Activity during test	Patient ambulatory; no exercise, eating, drinking (except water), smoking
Glucose load — adults	75 g anhydrous glucose
Glucose load — children	1.75 g/kg (max 75 g)
GDM	75 g; measure fasting, 1-hr, 2-hr
Non-pregnant measurements	Fasting + 2-hour samples ONLY (per ADA)
ADA preference	Not recommended for routine use — FPG or HbA1c preferred
Diagnostic cutoff	2-hour PG ≥ 200 mg/dL (confirm)

Factors That Affect OGTT Results

Factor	Effect
Medications (salicylates, diuretics, anticonvulsants, OCPs, corticosteroids)	Impair glucose tolerance → falsely elevated
GI problems (malabsorption, surgery, vomiting)	Alter absorption → unreliable
Endocrine dysfunction	Affects counter-regulatory hormones
Inadequate carb prep	Low-carb diet → impaired tolerance mimicking DM
Afternoon testing	Lower values → may miss DM

OGTT glucose load solution should NOT contain fructose — fructose interferes with hexokinase. Patient must eat normal/high carbs for 3 days before — low-carb diet falsely impairs glucose tolerance.

10. Ketone Bodies

Three Ketone Bodies

Ketone Body	% of Total	Clinical Note
β-hydroxybutyric acid	78%	LARGEST fraction. NOT detected by nitroprusside. Only enzymatic method. Important in DKA
Acetoacetic acid (acetoacetate)	20%	Detected by NITROPRUSSIDE; also enzymatic
Acetone	2%	Volatile — FRUITY BREATH. Detected by nitroprusside if glycerin added

Ketone Methods

Method	Details
NITROPRUSSIDE	NaFe(CN)₅NO + acetoacetate at alkaline pH → PURPLE. With glycerin → also detects acetone. Used in dipsticks, Acetest tablets. Does NOT detect β-hydroxybutyrate
Gerhardt’s Test (historical)	FeCl₃ + acetoacetate → red. Many interferences (salicylates)
ENZYMATIC (3-HBD)	3-hydroxybutyrate dehydrogenase. pH 7.0 → detects β-HB; pH 8.5–9.5 → detects acetoacetate. Adapted to automated analyzers
Specimen	FRESH serum or urine, tightly stoppered, analyzed IMMEDIATELY (volatile)
When to measure	Type 1 DM during: acute illness, stress, pregnancy, glucose >300, signs of ketoacidosis

CRITICAL TRAP: Nitroprusside detects ACETOACETATE (and acetone with glycerin) but DOES NOT detect β-hydroxybutyrate — the most abundant ketone (78%). A patient in severe DKA may have a FALSELY NEGATIVE or weakly positive nitroprusside test because β-HB dominates. Use the enzymatic 3-HBD method.

Causes of ketonemia: Type 1 DM, starvation/fasting, high-fat diets, prolonged vomiting, glycogen storage disease. Mechanism: low insulin/carb availability → unregulated lipolysis → β-oxidation → ketone bodies.

11. Microalbuminuria & Diabetic Nephropathy

Feature	Details
Clinical significance	Early marker of diabetic nephropathy — precedes clinical proteinuria by years. Aggressive glycemic control delays progression
Normal albumin excretion	< 30 mg/24 hrs or < 30 µg/mg creatinine
MICROALBUMINURIA	30–300 mg/24 hrs OR ACR 30–300 µg/mg creatinine
MACROALBUMINURIA	≥ 300 mg/24 hrs OR ACR ≥ 300 µg/mg
Diagnosis	Abnormal in 2 of 3 specimens within 3–6 month period
PREFERRED method	Random SPOT collection for ACR — convenient, no timed collection needed
Why spot + creatinine?	Creatinine corrects for urine dilution
24-hr collection	Burdensome; adds little accuracy
ADA recommendation	Annual urinary albumin assessment for ALL diabetic patients
False elevation	Exercise within 24 hrs, infection, fever, CHF, marked hyperglycemia, marked hypertension
Detection method	Immunoturbidimetry or immunonephelometry

SPOT collection + ACR is PREFERRED. The 24-hour collection is NOT preferred. Microalbuminuria = ACR 30–300 µg/mg. Macroalbuminuria = ACR ≥ 300 µg/mg.

12. Glycogen Storage Diseases & Inborn Errors

High-Yield Glycogen Storage Diseases

Type	Disease	Deficient Enzyme	Key Features
Type I	Von Gierke	Glucose-6-Phosphatase	MOST COMMON/SEVERE. Massive hepatomegaly, severe fasting hypoglycemia, ↑ lactate, ↑ uric acid, hypertriglyceridemia. Glucagon/epinephrine give NO glucose rise. Galactose tolerance test confirms (no glucose rise). AR
Type II	Pompe	Acid α-glucosidase (acid maltase)	Heart & skeletal muscle. Cardiomegaly, weakness. NO hypoglycemia. Infantile form: death <1 yr. Enzyme replacement available
Type III	Cori / Forbes	Glycogen debranching enzyme	Resembles Type I but: hyperglycemic response to galactose, lower urate & lactate, ↑ CK, AST/ALT
Type V	McArdle	Muscle Phosphorylase	Exercise cramps. NO hypoglycemia. Ischemic exercise: lactate does NOT increase (NH₃ ↑↑). ↑ resting CK. Respond to oral glucose
Type VI	Hers	Liver Phosphorylase	Benign; hepatomegaly, normal glycogen. Mild
Type VII	Tarui	Muscle Phosphofructokinase	Like McArdle + hemolysis (RBCs affected). Hyperbilirubinemia, pigmenturia

Galactosemia

Feature	Details
Most common deficiency	Galactose-1-phosphate uridyltransferase (GALT)
Diagnosis clue	Positive reducing substance + NEGATIVE glucose oxidase → non-glucose reducing sugar (galactose)
Symptoms	Failure to thrive on milk, vomiting, diarrhea, liver disease, cataracts, MR if untreated
Lab findings	Hypoglycemia, hyperbilirubinemia, galactose in blood/urine after milk
Confirmation	Erythrocyte GALT activity assay
Treatment	REMOVE galactose from diet immediately
Galactokinase deficiency	Milder — cataracts only (galactitol in lens)
Mechanism of hypoglycemia	Galactose inhibits glycogenolysis

Disorders of Fructose Metabolism

Disorder	Deficient Enzyme	Features
Essential Fructosuria	Fructokinase	BENIGN, asymptomatic. Fructose in urine; reducing substance positive
Hereditary Fructose Intolerance	Aldolase B (F-1-P aldolase)	Hypoglycemia + liver FAILURE after fructose. Treat: remove sucrose/fructose
F-1,6-Diphosphatase Deficiency	Fructose-1,6-bisphosphatase	Hypoglycemia, ketosis, lactic acidosis (impaired gluconeogenesis); apnea, hyperventilation

Von Gierke (Type I GSD): severe FASTING hypoglycemia + lactic acidosis + no glucose response to glucagon, epinephrine, OR galactose (G6Pase absent — can’t dephosphorylate). Confirm with liver biopsy for G6Pase activity.

13. Mind Maps — Visual Pattern Recognition

🧠 Mind Map 1: Glucose Methods — Decision Tree

Routine serum/plasma glucose
Hexokinase (UV, 340 nm) — Reference method
Urine glucose
Hexokinase or GDH-NAD — NOT glucose oxidase
Patient on icodextrin peritoneal dialysis (POC)
Hexokinase-based meter — NOT GDH-PQQ
High bilirubin / uric acid serum
Hexokinase preferred (unaffected) or polarographic glucose oxidase
Self-monitoring (SMBG)
Glucose oxidase or GDH-NAD POC meters
CSF glucose
Glucose oxidase or hexokinase — always compare to concurrent serum (~60% of plasma)

🧠 Mind Map 2: HbA1c Interference Pattern

Hemolytic anemia / sickle cell / G6PD def.
FALSE LOW — use fructosamine or CGM instead
Iron deficiency anemia
FALSE HIGH (older RBCs)
Cation exchange HPLC + Hgb SC
FALSE LOW — switch to affinity chromatography
Immunoassay + neonate Hgb F >10%
FALSE LOW
Cation exchange + Hgb F elevated
FALSE HIGH
Uremia / renal failure
FALSE LOW (carbamylation)
Lab temperature >22°C, cation exchange
FALSE LOW (temp-dependent elution)

🧠 Mind Map 3: Diabetes Complications Pattern

Glucose 500+ + ketones + Kussmaul + ↑ anion gap
DKA — Type 1 DM. Treat: fluids + insulin + electrolytes
Glucose 800–1000 + NO ketones + dehydration + altered MS
HHS — Type 2 DM. Treat: aggressive fluid replacement
Microalbuminuria (ACR 30–300) in DM
Early diabetic nephropathy — intensify glycemic + BP control
HbA1c 9% + FPG 90 mg/dL
High postprandial spikes — HbA1c reflects 24-hr average
HbA1c 5.5% + plasma glucose 250 chronically
Hemolytic disorder shortening RBC lifespan → false low HbA1c

🧠 Mind Map 4: Hypoglycemia Differential Pattern

Low glucose + HIGH insulin + HIGH C-peptide + HIGH proinsulin
INSULINOMA (endogenous hyperinsulinism)
Low glucose + HIGH insulin + LOW C-peptide
FACTITIOUS hypoglycemia (exogenous insulin)
Low glucose + HIGH insulin + HIGH C-peptide + drug detectable
Sulfonylurea-induced hypoglycemia
Low glucose + ↑ lactate + no glucagon response + hepatomegaly in infant
Von Gierke disease (Type I GSD)
Positive reducing substance + negative glucose oxidase in neonate
Galactosemia (after milk ingestion)
Postprandial symptoms 1–3 hrs after eating + glucose <45
Reactive hypoglycemia — demonstrate during spontaneous symptomatic episode

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15. The 25 Most Tested Points — Quick Reference

Glucose oxidase is SPECIFIC for β-D-glucose only; reacts with no other monosaccharide
Hexokinase = REFERENCE method; measures NADPH at 340 nm
Fluoride (gray top) inhibits ENOLASE — does NOT stop glycolysis in first 1–2 hrs. Separate cells within 30 min
Whole blood glucose ~10–12% LOWER than plasma (lower water content)
FPG is HIGHER in the morning — afternoon testing may MISS DM diagnosis
Nitroprusside detects ACETOACETATE (and acetone with glycerin) but NOT β-hydroxybutyrate
β-hydroxybutyrate = 78% of total ketones — most abundant
HbA1c reflects average glucose over past 2–3 months (120-day RBC lifespan)
Affinity chromatography is PREFERRED for HbA1c — NOT temp dependent, NOT affected by Hgb S, C, F
HbA1c is FALSELY LOW in hemolytic anemia, sickle cell, shortened RBC lifespan
ADA DM criteria: FPG ≥126, 2-hr OGTT ≥200, HbA1c ≥6.5%, Random ≥200 + symptoms
Prediabetes: IFG (FPG 100–125), IGT (2-hr 140–199), HbA1c 5.7–6.4%
DKA = Type 1: ketones POSITIVE, anion gap >16, Kussmaul, fruity breath (acetone)
HHS = Type 2: glucose >600–1000, NO ketones, severe dehydration, high mortality
INSULIN is the ONLY hormone that LOWERS blood glucose
GDM (any ONE): fasting ≥92, 1-hr ≥180, 2-hr ≥153 — LOWER thresholds than DM
Sucrose = ONLY common nonreducing disaccharide
Positive Clinitest + Negative glucose oxidase = non-glucose reducing substance (galactosemia in neonate)
Von Gierke (Type I GSD): G6Pase deficiency — NO glucose rise after glucagon, epinephrine, or galactose. Hypoglycemia + lactic acidosis + hepatomegaly
Insulinoma: glucose <55 + insulin ≥18 pmol/L + C-peptide ≥0.2 nmol/L + proinsulin ≥5 pmol/L
Factitious (exogenous insulin): HIGH insulin + LOW C-peptide
GDH-PQQ meters falsely ELEVATE glucose with icodextrin peritoneal dialysis (maltose cross-reacts)
Microalbuminuria = ACR 30–300 µg/mg; SPOT collection + ACR is PREFERRED
For urine glucose: HEXOKINASE or GDH-NAD — NOT glucose oxidase (uric acid → false LOW)
CSF glucose ~60% of concurrent plasma glucose — always measure both simultaneously

Quick Interpretation Table

Lab Finding	Clinical Interpretation
FPG 95 + HbA1c 9%	Poor long-term control; postprandial hyperglycemia or recent improvement
FPG 160 + moderate ketones + young patient	Likely Type 1 DM — confirm autoantibodies; insulin-dependent
FPG 200 + no ketones + obese adult + glucose 4+ urine	Likely Type 2 DM — HHS risk
Hypoglycemia + ↑ insulin + LOW C-peptide	Exogenous insulin (factitious)
Hypoglycemia + ↑ insulin + ↑ C-peptide	Insulinoma or sulfonylurea
Reducing substance + glucose oxidase neg + neonate on milk	Galactosemia — confirm with RBC transferase activity
Hypoglycemia + ↑ lactate + hepatomegaly + infant + no glucagon response	Von Gierke (Type I GSD)

16. Rapid Revision Sheet (Last 2 Hours Before Exam)

Glucose Methods

Hexokinase	Reference method \| Glucose + ATP → G6P \| G6PD → NADPH at 340 nm \| NOT affected by ascorbic acid or uric acid \| Affected by gross hemolysis & lipemia
Glucose Oxidase	β-D-glucose ONLY \| Peroxidase step: false LOW with uric acid, bilirubin, ascorbic acid \| Polarographic avoids interferences \| NOT for urine
GDH-PQQ	FALSE HIGH with maltose/icodextrin \| Dangerous in peritoneal dialysis \| Use GDH-NAD or hexokinase
Pre-analytical	Gray top = NaF inhibits enolase \| NOT immediate \| Separate within 30 min \| Whole blood 10–12% lower than plasma \| FBG higher in AM

HbA1c

Affinity Chromatography	PREFERRED \| NOT temp dependent \| NOT affected by Hgb S, C, F
Cation Exchange HPLC	Temp dependent \| Hgb S,C = false LOW \| Hgb F = false HIGH
Immunoassay (POC)	Hgb F >10% = false LOW
False LOW HbA1c	Hemolysis, sickle cell, short RBC lifespan, uremia
False HIGH HbA1c	Iron deficiency, longer RBC lifespan
ADA goals	Diagnostic: ≥6.5% \| Target: <7% \| Prediabetes: 5.7–6.4%

Diagnostic Numbers

Normal FPG	70–99 mg/dL
IFG (Prediabetes)	100–125 mg/dL
Diabetes FPG	≥126 mg/dL
Normal 2-hr OGTT	≤140 mg/dL
IGT (Prediabetes)	140–199 mg/dL
Diabetes 2-hr OGTT	≥200 mg/dL
GDM (any 1)	Fasting ≥92 \| 1-hr ≥180 \| 2-hr ≥153
Microalbuminuria	ACR 30–300 µg/mg creatinine
Macroalbuminuria	ACR ≥300 µg/mg
Hypoglycemia symptoms	<50–55 mg/dL
Insulinoma: insulin	≥18 pmol/L when glucose <55
Insulinoma: C-peptide	≥0.2 nmol/L

DKA vs HHS — Critical Distinction

DKA (Type 1)	Glucose 300–500 \| KETONES POSITIVE \| Anion gap >16 \| Kussmaul \| Fruity breath \| Low pH/HCO₃ \| High K⁺ (total body K⁺ low)
HHS (Type 2)	Glucose >600–1000 \| KETONES ABSENT \| Minimal acidosis \| Severe dehydration \| Osmolality >320 \| High BUN/Cr
Why no ketones in HHS	Residual insulin suppresses lipolysis
Ketone rule	Nitroprusside = acetoacetate only. β-HB (78%) NOT detected. Use enzymatic 3-HBD

Hypoglycemia Patterns

↑ insulin + ↑ C-peptide	Insulinoma or sulfonylurea — check drug levels
↑ insulin + LOW C-peptide	Exogenous insulin (factitious)
No glucagon response + hepatomegaly + lactic acidosis	Von Gierke (Type I GSD)
Reducing substance + neg glucose oxidase in neonate	Galactosemia
Whipple’s triad	Low glucose + symptoms + relief with glucose

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