Blood Tests for Insulin Resistance
Order a fasting plasma glucose (FPG) and fasting insulin level to calculate HOMA-IR (Homeostasis Model Assessment of Insulin Resistance), which is the most practical and validated method for assessing insulin resistance in clinical practice. 1, 2, 3
Primary Testing Approach
HOMA-IR Calculation
- The HOMA-IR formula is: (fasting insulin in μU/mL × fasting glucose in mmol/L) / 22.5 2, 3
- A HOMA-IR value ≥2.35-2.6 generally indicates insulin resistance in adults, though this cutoff may vary by BMI category 2, 4
- Higher BMI categories require higher HOMA-IR cutoff values for optimal discrimination of metabolic syndrome 2
Required Blood Tests
- Fasting plasma glucose (FPG): Must be drawn after 8-14 hours of fasting using venous plasma samples with enzymatic assay techniques 5
- Fasting insulin level: Drawn simultaneously with FPG to enable HOMA-IR calculation 1, 3
Alternative Testing Methods
QUICKI Index
- The Quantitative Insulin Sensitivity Check Index (QUICKI) = 1/(log insulin + log glucose in mg/dL) provides another validated approach 4, 6
- QUICKI values <0.357 typically indicate insulin resistance with high specificity (0.91) and sensitivity (0.75) 4, 6
- QUICKI demonstrates lower dispersion variance and better discrimination capacity compared to HOMA-IR 6
HbA1c-Based Estimation
- HbA1c can substitute for fasting glucose in HOMA calculations when fasting samples are impractical, using estimated average glucose (eAG) derived from HbA1c 1
- This approach enables home sampling and avoids pre-analytical errors associated with glucose measurements 1
- HbA1c-derived eAG-based HOMA correlates very well with standard HOMA-IR (r² = 0.9972) 1
Clinical Context and Interpretation
When to Test
- Screen adults with BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans) who have additional risk factors including hypertension, dyslipidemia, polycystic ovary syndrome, acanthosis nigricans, or family history of diabetes 5
- Consider testing in patients with clinical signs of insulin resistance such as acanthosis nigricans, central obesity, or metabolic syndrome features 5
Correlation with Clinical Parameters
- HOMA-IR correlates positively with BMI, waist circumference, visceral fat area, triglycerides, and blood pressure 3
- HOMA-IR correlates negatively with HDL cholesterol and adiponectin levels 3
- These correlations validate HOMA-IR as a marker of the metabolic syndrome cluster 3
Important Caveats
Limitations of HOMA-IR
- HOMA-IR is most accurate when fasting glucose is between 4.44-6.67 mmol/L (80-120 mg/dL) 1
- The test assumes steady-state conditions and may be less reliable in patients with marked hyperglycemia or hypoglycemia 3
- Different populations may require population-specific cutoff values due to ethnic and metabolic variations 2, 6
Laboratory Considerations
- Significant interlaboratory variation exists in insulin assays, so each laboratory should establish its own reference range with appropriate control groups 6
- Venous plasma samples are preferred over capillary or whole blood samples for standardization 5
- Pre-analytical errors are common with glucose measurements, making HbA1c-based calculations attractive alternatives 1
Not a Diagnostic Test for Diabetes
- HOMA-IR assesses insulin resistance, not diabetes diagnosis 5
- For diabetes diagnosis, use FPG ≥126 mg/dL (7.0 mmol/L) on two occasions, 2-hour OGTT ≥200 mg/dL (11.1 mmol/L), or HbA1c ≥6.5% 5, 7
- Insulin resistance often precedes overt diabetes by years and represents a therapeutic target for prevention 5, 8