Testing Insulin Levels in Suspected Diabetes
Insulin level testing is not required or recommended for diagnosing diabetes mellitus. The diagnosis should be made using glucose-based criteria (fasting plasma glucose, oral glucose tolerance test, HbA1c, or random plasma glucose with symptoms) rather than insulin measurements 1.
Standard Diagnostic Tests for Diabetes
The following tests are recommended for diagnosing diabetes, none of which involve measuring insulin levels 1:
- Fasting Plasma Glucose (FPG) ≥126 mg/dL (7.0 mmol/L) after at least 8 hours of fasting 1, 2
- HbA1c ≥6.5% using an NGSP-certified laboratory method 1
- 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during a 75-g oral glucose tolerance test 1, 3
- Random plasma glucose ≥200 mg/dL (11.1 mmol/L) in patients with classic hyperglycemic symptoms 1
In the absence of unequivocal hyperglycemia, results must be confirmed by repeat testing on a different day 1.
Why Insulin Testing Is Not Used for Diagnosis
Research demonstrates that plasma insulin assays are not widely used in clinical practice for diabetes diagnosis and provide no decisive help in distinguishing between various forms of diabetes (type 2, slow type 1, MODY, or mitochondrial defects) 4. The measurement of plasma insulin has not been shown to help adapt treatment in diabetic patients, except potentially for determining the need for insulin therapy 4.
Limited Clinical Scenarios Where Insulin/C-Peptide May Be Measured
While not part of standard diagnostic criteria, insulin or C-peptide measurement may occasionally be considered in specific clinical contexts:
- Distinguishing type 1 from type 2 diabetes when clinical presentation is unclear: Low or undetectable C-peptide levels suggest advanced beta-cell destruction in type 1 diabetes 1
- Assessing residual beta-cell function in established diabetes, where C-peptide <1.51 ng/mL has a 96% negative predictive value for type 1 diabetes diagnosis 1
- Research settings to assess beta-cell sensitivity or insulin secretion kinetics, but these are not clinically actionable for routine diagnosis 4
Essential Pre-Treatment Laboratory Evaluation
Once diabetes is diagnosed, the following tests are recommended before initiating therapy (not for diagnosis):
- HbA1c at least twice yearly if targets are met, or quarterly if therapy is altered 5
- Serum creatinine and eGFR to assess baseline renal function 5
- Urine albumin-to-creatinine ratio annually starting at diagnosis 5
- Lipid panel and blood pressure at each visit 5
Common Pitfalls to Avoid
- Do not order insulin levels to diagnose diabetes—this is not part of any guideline-recommended diagnostic algorithm 1
- Do not use glucometer readings alone for diagnosis—abnormal results must be confirmed with standardized laboratory testing 2
- Do not rely solely on HbA1c in conditions with increased red blood cell turnover (sickle cell disease, pregnancy, recent blood loss); use plasma glucose criteria instead 5, 2
- Do not delay confirmation testing—if two different tests from the same sample are above diagnostic thresholds, repeat testing in 3-6 months after discussing symptoms with the patient 1