Glucose & insulin
Fasting glucoseFPG
Plasma glucose after ≥8 h fast; the simplest dysglycemia screen.
Sample
Plasma (fluoride tube preferred to prevent in-vitro glycolysis)
Fasting
≥8 hours, water permitted
Clinical reference range
70 to 99 mg/dL
ADA normal
Conv: 70–99 mg/dLSI: 3.9–5.5 mmol/LFunc: 75–90 mg/dL
Biology
What it measures
Reflects basal hepatic glucose output balanced against insulin-mediated suppression. Rises hours before postprandial glucose in early type 2 diabetes.
Clinical use
Why we order it
Diagnose prediabetes (100–125) and diabetes (≥126 on two occasions); monitor known diabetes.
Lens · Clinical
Interpretation by lens
Clinical interpretation
ADA: FPG ≥126 on two occasions OR HbA1c ≥6.5% OR 2-h OGTT ≥200 OR random ≥200 + symptoms.
Functional interpretation
Functional optimal is 75–90; values 90–99 with rising A1c flag early insulin resistance — start lifestyle intervention before crossing the diabetes threshold.
Research note
Continuous glucose monitoring shows that mean glucose, time-in-range, and glucose variability all add prognostic information beyond fasting glucose.
Differential
Causes of abnormal values
Causes of HIGH
- ↑Prediabetes / type 2 diabetes
- ↑Stress hyperglycemia (sepsis, MI, steroids)
- ↑Cushing's, acromegaly
- ↑Pancreatic disease
- ↑Drugs: glucocorticoids, atypical antipsychotics, thiazides, tacrolimus
Causes of LOW
- ↓Insulin/sulfonylurea overdose
- ↓Insulinoma (paired insulin and C-peptide elevated)
- ↓Adrenal insufficiency
- ↓Sepsis (late), liver failure
- ↓Malnutrition, alcohol
Pitfalls
Pre-analytic & interpretation traps
- !Non-fluoride tubes lose ~5% glucose per hour at room temperature — falsely lowers result.
- !Stress hyperglycemia in critical illness is a marker of severity, not necessarily diabetes.
Follow-up orders
Logical next-step labs
Evidence-graded claims
What the data says
A
ADA fasting glucose thresholds are appropriate for diabetes diagnosis
Evidence basis is microvascular complication risk; thresholds are pragmatic.
D
Fasting glucose 90–99 with normal A1c is harmless
Rising fasting glucose in this range correlates with insulin resistance and future diabetes risk.
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