Glucose and Insulin Unit Conversions and Clinical Thresholds

Glucose Conversion

• Blood glucose values expressed in mg/dL can be converted to mmol/L by dividing by 18 [​@1​].
• This conversion factor is consistently applied across major clinical guidelines and laboratory practice [​@1​, @2​].
• The reverse conversion (mmol/L → mg/dL) is performed by multiplying the mmol/L value by 18 [​@2​].

Glucose Clinical Thresholds

• Hypoglycemia diagnostic threshold: 0.45 g/L = 45 mg/dL = 2.5 mmol/L [​@3​, @4​].
• Alternative hypoglycemia threshold: 0.55 g/L = 55 mg/dL = 3.0 mmol/L [​@3​, @5​].
• Diabetes diagnostic threshold: 126 mg/dL = 7.0 mmol/L [​@6​].
• Hyperglycemia treatment threshold (hospitalized patients): 180 mg/dL = 10 mmol/L [​@2​].

Insulin Conversion

• The conversion from µU/mL (mIU/L) to pmol/L varies between 6.0 – 7.5 depending on the calibration standard; recent calibrations adopt a factor of 6.0 (3 mIU/L = 18 pmol/L) [​@7​, @8​].
• No single universally accepted conversion factor exists because of assay‑specific variability; the specific calibration standard (e.g., IRP 66/304 or 83/500) must be reported [​@7​].

Insulin Clinical Thresholds

• Inappropriate endogenous insulin secretion: Insulinemia ≥ 3 mIU/L (≈18 pmol/L) during hypoglycemia (glucose ≤ 45 mg/dL) yields 100 % specificity [​@4​, @9​, @10​].

C‑Peptide Conversion

• C‑peptide concentrations can be converted from pmol/L to ng/mL by multiplying by 0.003 [​@4​, @9​].

C‑Peptide Clinical Thresholds

• Inappropriate endogenous insulin secretion: C‑peptide ≥ 0.6 ng/mL (≈200 pmol/L) during hypoglycemia (glucose ≤ 45 mg/dL) provides ≈100 % sensitivity and specificity [​@4​, @9​].

Practical Considerations for Insulin Measurement

• Hemolysis must be strictly avoided because erythrocyte enzymes degrade insulin, causing under‑estimation in immunometric assays [​@8​, @7​].
• Anti‑insulin antibodies can interfere with assay results; when present, free insulin should be measured [​@8​, @7​].
• Heparinized plasma is not recommended for insulin assays, as it yields lower insulin concentrations than serum [​@8​].

Standardization Efforts

• The American Association of Clinical Chemistry (AACC) and the American Diabetes Association (ADA) have called for harmonization of insulin assays, acknowledging the current lack of standardization [​@7​].