Hypoglycemia is diagnosed by confirming three things together: you have symptoms, your blood sugar is low at the time of those symptoms, and your symptoms go away once your blood sugar is corrected. This three-part check, called Whipple’s triad, is the foundation of every hypoglycemia diagnosis. The specific tests and thresholds used depend on whether you have diabetes or whether your doctor is investigating unexplained low blood sugar in someone without diabetes.
The Three Criteria That Confirm a Diagnosis
A single low blood sugar reading on its own isn’t enough to diagnose a hypoglycemic disorder. Doctors look for all three parts of Whipple’s triad occurring together: recognizable symptoms of low blood sugar, a confirmed low glucose reading at the time those symptoms happen, and prompt relief of symptoms once glucose is given. If any one of those three pieces is missing, the diagnosis isn’t considered confirmed.
This matters because some people, particularly women, can have fasting glucose levels as low as 50 mg/dL without any symptoms at all. A number alone doesn’t tell the full story.
Blood Sugar Thresholds That Define Hypoglycemia
The cutoff for “low” depends on context. For people with diabetes who take insulin or certain glucose-lowering medications, hypoglycemia is defined as a blood glucose at or below 70 mg/dL. Severity is then graded in three levels:
- Level 1 (mild): Blood glucose below 70 mg/dL but at or above 54 mg/dL
- Level 2 (moderate): Blood glucose below 54 mg/dL
- Level 3 (severe): Blood sugar low enough that you need help from another person because of confusion, loss of coordination, or loss of consciousness
For people who don’t have diabetes, the diagnostic threshold is stricter. A plasma glucose below 55 mg/dL, measured at the same time symptoms are present, is the standard cutoff. Research under controlled conditions shows that the body’s stress response (shakiness, sweating, rapid heartbeat) typically kicks in around 60 mg/dL, while brain-related symptoms like confusion, difficulty concentrating, and visual changes appear around 50 mg/dL.
Testing for People With Diabetes
If you have diabetes and use insulin or medications that lower blood sugar, diagnosis is usually straightforward. A fingerstick glucose meter or a continuous glucose monitor (CGM) can capture the low reading when symptoms occur. The Endocrine Society recommends CGM over traditional fingerstick monitoring for people with type 1 diabetes on multiple daily injections, in part because CGMs track glucose around the clock and can catch lows that happen overnight or between meals when you might not be testing.
Your doctor will also review your medication regimen, meal timing, and activity levels to identify patterns. Sometimes the fix is as simple as adjusting a dose or switching to a different type of insulin that carries a lower risk of causing lows.
The Supervised Fasting Test
When someone without diabetes has repeated episodes of unexplained low blood sugar, the most definitive diagnostic tool is a supervised fast that can last up to 72 hours. You’re admitted to the hospital, given a meal, and then monitored as you stop eating. Blood samples are drawn every six hours to measure glucose, insulin, C-peptide (a marker of how much insulin your body is producing), and several hormones including cortisol and growth hormone.
If your blood sugar drops below 60 mg/dL during the fast, blood draws increase to every two hours. The test ends when Whipple’s triad is confirmed, when glucose falls to 45 mg/dL or below, or when 72 hours have passed without an episode. At the end, a small dose of glucagon is given and your glucose response is measured over the next 30 minutes to provide additional diagnostic information.
This test is particularly useful for detecting insulinomas, which are small tumors of the pancreas that produce excess insulin. Most insulinomas will cause a detectable drop in blood sugar within 48 hours of fasting, though some require the full 72 hours.
What Blood Markers Reveal About the Cause
When blood sugar drops below 55 mg/dL during testing, doctors measure insulin, C-peptide, and proinsulin levels simultaneously. The pattern of these markers points to the underlying cause.
If all three are elevated while blood sugar is low, that suggests your body is making too much insulin on its own, pointing toward an insulinoma or a similar condition. The widely used thresholds for this pattern are insulin at or above 3 µIU/mL, C-peptide at or above 0.6 ng/mL, and proinsulin at or above 5 pmol/L, all measured alongside a glucose of 54 mg/dL or less.
If insulin is high but C-peptide is low, that points to an external source of insulin, such as accidental or intentional injection. This distinction between the body’s own insulin production and insulin coming from outside the body is one of the most important diagnostic steps.
Testing for Reactive Hypoglycemia
Some people experience low blood sugar not during fasting but after eating, typically two to five hours after a meal. This is called reactive or postprandial hypoglycemia, and it requires a different test. A mixed meal tolerance test involves eating a standard meal and then having blood drawn for glucose, insulin, and C-peptide at regular intervals: 30, 60, 90, 120, 150, 180, 240, 270, and 300 minutes afterward.
There is no universal consensus on the exact glucose threshold that confirms reactive hypoglycemia. In practice, doctors interpret the results the same way they would during a fasting test: symptoms consistent with hypoglycemia alongside a glucose reading below about 54 mg/dL (3.0 mmol/L) is considered a positive finding. This test is also sometimes used alongside the fasting test when an insulinoma is suspected, since some tumors cause drops primarily after meals.
Ruling Out Other Causes
Before concluding that you have a primary hypoglycemic disorder, your doctor needs to rule out a long list of other possibilities. Many non-diabetes medications can cause low blood sugar, including certain antibiotics (quinolones), beta-blockers, ACE inhibitors, and lithium. Alcohol is another common culprit, especially on an empty stomach.
Organ failure, particularly liver or kidney disease, can impair the body’s ability to maintain normal glucose levels. Severe infections, adrenal insufficiency, and pituitary problems can all do the same. Hypoglycemia is also increasingly recognized after bariatric surgery, sometimes appearing months or years after the procedure. In each of these cases, treating the underlying condition or stopping the offending medication is the first step, and specialized hypoglycemia testing may not be needed if the cause is clear.
The Role of Continuous Glucose Monitors
CGMs, which use a small sensor under the skin to measure glucose every few minutes, are well established for managing diabetes. Their role in diagnosing non-diabetic hypoglycemia is still limited but growing. Case reports describe CGMs being used to detect patterns in people with inborn metabolic conditions and in other specific clinical situations.
One important caveat: CGMs are less accurate at low glucose levels than at normal or high levels. A reading showing a low on a CGM should ideally be confirmed with a standard blood draw before making diagnostic decisions. For people with diabetes who already wear a CGM, it remains one of the best tools for catching and documenting hypoglycemic episodes as they happen in real life, which gives your doctor concrete data to work with.