Calculating Insulin Resistance Indices Using mg/dL and μU/mL
To calculate insulin resistance indices using fasting glucose in mg/dL and fasting insulin in μU/mL, apply these formulas directly without unit conversion: HOMA-IR = (glucose × insulin)/405, QUICKI = 1/[log(insulin) + log(glucose)], G/I ratio = glucose/insulin, and HOMA-%S = (20 × insulin)/(glucose - 3.5).
Standard Formulas for mg/dL Units
When using fasting glucose in mg/dL and fasting insulin in μU/mL (which equals mIU/L), the calculations are straightforward:
HOMA-IR (Homeostasis Model Assessment of Insulin Resistance)
- Formula: HOMA-IR = (Fasting Glucose in mg/dL × Fasting Insulin in μU/mL) / 405
- Reference range: Normal values <2.0 in adults 1
- Interpretation: Values ≥2.86 suggest insulin resistance 1
- This is the most widely validated index with strong correlation to gold-standard clamp studies 2, 3
QUICKI (Quantitative Insulin Sensitivity Check Index)
- Formula: QUICKI = 1 / [log(Fasting Insulin in μU/mL) + log(Fasting Glucose in mg/dL)]
- Reference range: Normal values in healthy adults = 0.366 ± 0.029 4
- Interpretation: Values <0.357 indicate insulin resistance 4
- Note that logarithms are base 10 (common logarithm) 3
G/I Ratio (Glucose-to-Insulin Ratio)
- Formula: G/I Ratio = Fasting Glucose in mg/dL / Fasting Insulin in μU/mL
- Reference range: Values <4.5 suggest insulin resistance in PCOS women 5
- Interpretation: A cut-off of ≤6 shows 73.3% sensitivity and 74.7% specificity for detecting abnormal glucose tolerance 6
HOMA-%S (Homeostasis Model Assessment of Insulin Sensitivity)
- Formula: HOMA-%S = (20 × Fasting Insulin in μU/mL) / (Fasting Glucose in mg/dL - 3.5)
- This represents insulin sensitivity as a percentage of normal
- Lower values indicate greater insulin resistance
Practical Considerations
Sample requirements: Obtain fasting samples after 8-14 hours of fasting 7. The patient should avoid smoking and remain seated during collection.
Which index to use: HOMA-IR demonstrates the best overall performance for clinical assessment 2, 8. In one comparative study, HOMA-IR identified 69% of PCOS women with insulin resistance, while fasting insulin alone missed 8 subjects with insulin resistance and endocrine abnormalities 2. For metabolic syndrome screening, HOMA-IR combined with triglycerides (HOMA-TG index) shows the highest diagnostic accuracy 8.
Sex differences: Despite statistically significant differences between men and women, sex-specific reference intervals are not clinically necessary—the same reference ranges apply to both sexes 1.
Clinical context matters: QUICKI and G/I ratio show lower diagnostic accuracy for metabolic syndrome compared to HOMA-IR 8. However, the G/I ratio performs well specifically in PCOS populations for detecting abnormal glucose tolerance 6.
Common Pitfalls to Avoid
- Do not convert units: These formulas are designed for mg/dL and μU/mL directly. Converting to mmol/L requires different divisors (22.5 instead of 405 for HOMA-IR).
- Insulin assay variability: Establish laboratory-specific reference ranges when possible, as significant interlaboratory variation exists in insulin measurements 4.
- Timing sensitivity: Ensure true fasting state; postprandial samples invalidate these calculations.
- Population differences: Reference ranges may vary by ethnicity and age. The G/I ratio cut-off of <4.5 was validated specifically in obese non-Hispanic white PCOS women 5.