Fasting insulin levels are often tested repeatedly, despite not being a recommended screening or monitoring test for insulin resistance (IR). Many patients mistakenly believe they have insulin resistance and avoid carbohydrates based on high-normal fasting insulin levels, even when they have no other clinical or biochemical evidence of IR. Diagnosing insulin resistance based on fasting insulin levels alone is almost like creating a condition that doesn’t exist and providing unnecessary treatment.
Key Principles to Consider
1️⃣ Validation of the Test: Is the test validated to actually make the diagnosis? The gold standard for diagnosing insulin resistance is an insulin clamp test, and fasting insulin levels have only a weak association with these results. Prospective studies have shown that insulin levels do not predict metabolic syndrome or cardiovascular disease.
2️⃣ Intra-Individual Variation: How much does the result vary in the same person on different days? There is wide intra-individual variation in fasting insulin levels, which impacts interpretation.
3️⃣ Intra-Lab Variation: How much do results vary between laboratories and types of tests (assays)? Insulin levels can vary widely between assays, as can testosterone and fT3 and fT4.
4️⃣ Accuracy of Reference Ranges: How accurate are the stated reference ranges? If the reference ranges are based on only a few small cohorts, they do not truly represent the real population range.
These concepts are often poorly understood or considered by those who are not medically trained but market themselves as qualified to interpret blood results.
Appropriate Use of Insulin Levels
Insulin levels should almost only be tested when working up hypoglycemia. As an MJA paper states, “Interpreting fasting insulin levels may be considered like wrestling a column of smoke.” Do not assume you have insulin resistance based on fasting insulin levels alone. SHBG, triglycerides, fasting glucose, and a thorough history and examination are more accurate and appropriate.