Lab Assessment of Abnormal Glucose Metabolism Part 1

Tests for evaluating glucose metabolism.

Fasting glucose, random glucose, OGTT

Fasting plasma glucose fasting requirement.

At least 8 hours

Fasting glucose reference range <30 days.

2.5–5.5 mmol/L

Fasting glucose reference range ≥30 days.

3.3–6.0 mmol/L

Fasting glucose critical values <30 days.

Fasting glucose critical values ≥30 days.

Random glucose reference range <30 days.

2.5–11.0 mmol/L

Random glucose reference range ≥30 days.

3.3–11.0 mmol/L

Random glucose critical values <30 days.

Random glucose critical values ≥30 days.

OGTT fasting glucose rule.

If fasting glucose >7.8 mmol/L, do not proceed; order fasting glucose

OGTT glucose drink adult dose.

75 g

OGTT glucose drink child dose.

1.75 g/kg

OGTT ingestion requirement.

Drink must be consumed within 5 minutes

OGTT vomiting rule.

If vomiting occurs within 30 minutes, discontinue test

Diabetes diagnostic criteria.

If asymptomatic, require second confirmatory test; if symptomatic, no confirmation needed

Define prediabetes.

Abnormal glucose tests not meeting diabetes criteria

Prediabetes markers.

IFG, IGT, HbA1c 6.0–6.4%

GDM screening timing.

24–28 weeks gestation

GDM screening test.

50 g glucose drink, non‑fasting, 1‑hour plasma glucose

GDM confirmatory test.

2‑hour 75 g OGTT if screening abnormal

Define glycosylation.

Attachment of carbohydrates (often oligosaccharides) to proteins

Define glycation.

Attachment of monosaccharides (usually glucose) to amino groups of proteins

HbA1c formation.

Glucose reacts with N‑terminal valine of hemoglobin A β‑chains forming stable ketoamine

Stability of HbA1c.

Remains for RBC lifespan (~120 days)

HbA1c reference range.

4.3–5.9%

Two factors determining HbA1c.

Blood glucose concentration and RBC lifespan

Conditions misleading for HbA1c.

Hemoglobinopathies, hemolytic anemia, iron deficiency

Effect of shortened RBC lifespan on HbA1c.

Falsely decreased HbA1c

Effect of blood loss on HbA1c.

Falsely decreased due to more young RBCs

HbA1c measurement principle.

Separate glycated hemoglobin from non‑glycated hemoglobin

HbA1c result reporting.

Percentage of total hemoglobin

HbA1c chemical analysis methods.

Enzymatic, spectrophotometric

HbA1c specimen requirements.

EDTA whole blood

Define fructosamine.

Glycated plasma proteins (e.g., albumin)

Fructosamine time window.

Reflects average glucose over 2–3 weeks

Fructosamine clinical use.

Useful when HbA1c inaccurate or for monitoring rapid treatment changes