25 Sept 2026
Why Your Doctor Will Not Order a Fasting Insulin Test
A man in his 40s came to my emergency department on a Sunday morning with an abscess. We drained it, checked his blood sugar because of where the abscess was, and it came back at 440. He went home with antibiotics, metformin, and a diagnosis of type 2 diabetes he had no idea he had.
A number like that takes years to arrive. For most of those years, a routine blood test would have looked normal. This episode is about why that happens, and about the one test on your panel that does not care what you had for breakfast.
This is episode 2 of Lab Results Explained, the reference series in Overheard in the Emergency Room. I walk through the physiology of insulin and the two very different ways it fails, the 3 tests used to diagnose diabetes and what each one physically measures, the exact diagnostic thresholds, and where the US Preventive Services Task Force and the American Diabetes Association agree and differ on who should be screened.
I also take on the loudest claim in online metabolic health, which is that your doctor is deliberately withholding a fasting insulin test that would have caught this years earlier. The biology behind that claim is correct. The conclusion is not, and I explain exactly why using the assay standardization data.
And then the part I actually made this episode for: the personal fat threshold, the DiRECT trial, and what remission really means for a condition most people are told is permanent.
KEY TAKEAWAYS
• An A1c reflects roughly 3 months of blood sugar,because red blood cells live about 120 days. It does not require fasting.
• Diabetes thresholds: fasting glucose 126 mg/dL orhigher, A1c 6.5 percent or higher, or a 2-hour glucose of 200 or higher. Prediabetes: 100 to 125, 5.7 to 6.4 percent, or 140 to 199.
• Unless the hyperglycemia is unequivocal, a diagnosis needs a confirmatory test. One abnormal number is a reason to test again, not to panic.
• The USPSTF recommends screening adults 35 to 70 withoverweight or obesity. The ADA screens everyone from 45, and earlier with risk factors.
• Anemia, kidney failure, pregnancy, recent transfusion, and hemoglobin variants can all make an A1c unreliable. Use glucose criteria instead.
• Fasting insulin is a real physiological quantity and a legitimate research tool, but assay variation between labs ran from 12 to 66 percent, no society has ratified a diagnostic cutoff, and HOMA-IR is not recommended outside research.
• In DiRECT, 46 percent of the intervention group were in remission at 12 months versus 4 percent of controls, and 86 percent of those who lost 15 kg or more went into remission.
• The highest-yield action for most listeners is not atest at all. It is having a primary care physician who is looking.
DISCLAIMER
This episode is for educational purposes only. It does not provide medical advice or establish a physician-patient relationship. The case discussed has been deidentified. If you have symptoms concerning for a medical condition, please seek care from a qualified clinician. All sources are available at DrCois.com, with no affiliate links.
