Decoding the Diagnosis: Tests and Genetics

Confirming a diagnosis of HNF4A-related hyperinsulinism (CHI) is like solving a medical puzzle. Because the symptoms of low blood sugar can look the same regardless of the cause, doctors must catch the body “in the act” of mismanaging sugar to prove that inappropriate insulin action is the culprit.

The “Critical Sample”: Catching the Evidence

The most important step in diagnosis is obtaining a critical blood sample. This is a group of tests drawn at the exact moment your baby’s blood sugar is low. Treatment must never be delayed to obtain a sample, but when safely possible, hospitals follow strict protocols (often drawing the sample when lab plasma glucose is below 50 mg/dL or 2.8 mmol/L, though centers vary) ^[1][2].

By testing the blood during a “low,” doctors can see how the body is reacting to the crisis. They look for a pattern in several key biomarkers (biological clues) ^[2][3]:

• Insulin and C-peptide: In a healthy body, these should be suppressed to very low levels when sugar is low. If they are inappropriately detectable, it indicates the pancreas is pumping out insulin when it shouldn’t ^[3].
• Beta-hydroxybutyrate (Ketones): These are backup fuels the brain uses when sugar is low. Insulin blocks the production of ketones, so an unsuppressed insulin level leads to inappropriately low ketones during hypoglycemia ^[3][2].
• Free Fatty Acids (FFA): Like ketones, these should rise during a sugar crash as the body tries to burn fat for energy. If they are low, it suggests insulin is trapping fat in stores ^[3][2].

The Undetectable Insulin Pitfall

A common point of confusion is when a baby’s insulin level comes back as “undetectable.” You might wonder how it can be hyperinsulinism if there is no insulin found. However, research shows that in about 18% of confirmed CHI cases, the insulin level appears undetectable on standard assays due to testing limits ^[3]. An undetectable insulin result does not rule out hyperinsulinism by itself. Doctors rely on the whole pattern—especially C-peptide (a byproduct of insulin production that lasts longer in the blood) and the suppression of ketones—to make the diagnosis ^[3][4].

The Glucagon Stimulation Test

Under specialist supervision, doctors may perform a glucagon stimulation test. Glucagon is a hormone that tells the liver to release stored sugar (glycogen).

Normally, if a baby has been low for a while, they will have used up all their stored sugar, and a shot of glucagon won’t do much. However, because insulin prevents the liver from releasing those stores, a baby with CHI will have a “hidden” supply of sugar. If their blood sugar jumps up significantly (usually by 30 mg/dL or more) after a glucagon injection, it strongly supports the diagnosis of CHI ^[3]. A poor response, however, does not independently rule out the condition.

Genetic Testing: Finding the Molecular Cause

While blood and urine tests confirm that the body is producing too much insulin, genetic testing identifies the molecular cause. Testing for the HNF4A mutation helps distinguish permanent CHI from “transient” forms (like those seen in infants of diabetic mothers) ^[5][6].

However, genetic testing has limitations. A negative genetic panel does not exclude CHI (as not all genes causing CHI have been discovered), and finding a “variant of uncertain significance” (VUS) does not firmly establish HNF4A disease. Your results should always be reviewed by a clinical geneticist or genetic counselor.

Cascade Testing: A Family Affair

Because pathogenic HNF4A mutations are autosomal dominant, if a parent is confirmed to carry the variant, there is a 50% risk of passing it to each pregnancy ^[7]. This leads to the practice of cascade testing, where family members are offered testing after appropriate genetic counseling.

It is common for a parent to discover they carry the mutation even if they never had low blood sugar as a baby. Often, these parents find out that their early-onset “Type 2 diabetes” was actually HNF4A-MODY all along ^[8][6]. Identifying this can significantly improve the parent’s own diabetes management ^[9]. However, no family member should change their diabetes medication based simply on an infant’s diagnosis; they must wait for their own molecular testing and specialist consultation.