Can Premature Babies Have a False-Positive CAH Screen?

Receiving a call about an abnormal newborn screen while your baby is in the Neonatal Intensive Care Unit (NICU) is frightening. If your premature baby has a positive screen for Congenital Adrenal Hyperplasia (CAH), it is important to know that a screening result is not a diagnosis.

In premature, low-birth-weight, or critically ill infants, this test frequently flags a positive result even when the baby does not have CAH (a false-positive screen) ^[1][2]. However, because true classic CAH can cause a life-threatening adrenal crisis in the first weeks of life, a positive screen is never ignored and always requires prompt evaluation by your care team ^[3].

What This Result Means Right Now

• A positive screen simply means more testing is needed to find out if your baby actually has CAH or if the result was temporarily influenced by their prematurity.
• Follow your NICU team’s urgent follow-up plan. Do not wait for a repeat screen if the team advises immediate diagnostic testing.
• Warning Signs: Even if a false positive is strongly suspected, tell your medical team immediately if your baby experiences poor feeding, repeated vomiting, unusual sleepiness, weakness, dehydration (fewer wet diapers), or worsening illness ^[3]. Some babies show no early signs, so never wait for symptoms to complete the follow-up testing.

Why do false-positive screens happen in premature babies?

The standard newborn screen for CAH measures a hormone building block called 17-OHP (17-hydroxyprogesterone) ^[4]. In true CAH, a genetic enzyme deficiency prevents the adrenal glands from making essential hormones like cortisol (which helps the body respond to stress) and sometimes aldosterone (which balances salt and water). As a result, the 17-OHP building block backs up and reaches very high levels.

However, premature babies often have elevated 17-OHP for reasons entirely unrelated to CAH:

• Immature Adrenal Glands: A premature baby’s adrenal glands (small organs sitting on top of the kidneys that produce essential hormones) are still developing. Their early steroid hormone profiles naturally look different from those of full-term babies, leading to higher baseline levels of 17-OHP ^[4][5]. In babies without CAH, these levels generally decrease as they mature ^[4].
• Physical Stress: Illnesses, breathing difficulties, or the physical stress of an early delivery can cause a tiny baby’s body to temporarily spike 17-OHP levels ^[6][7].
• Test Limitations: The initial screening test (called an immunoassay) aims to maximize sensitivity to catch as many true cases of CAH as possible ^[8]. However, its testing chemicals can cross-react with normal physiological steroid precursors that are naturally abundant in premature babies ^[9]. This makes the measured 17-OHP level look artificially higher than it actually is.

Because 17-OHP naturally increases as gestational age and birth weight decrease, standard screening cutoffs will often flag perfectly healthy premature babies ^[1][2].

How do doctors clarify the result?

When a screen is positive, the next steps depend on the specific 17-OHP value, your baby’s gestational age, their birth weight, and their clinical health ^[10][11]. The care team, often alongside a pediatric endocrinologist (a hormone specialist), will determine the safest follow-up plan:

• Urgent Blood Tests and Monitoring: If the screening levels are markedly high or the baby is unwell, doctors will immediately draw blood from a vein to measure hormones accurately. They will also closely monitor electrolytes (sodium and potassium), glucose, and hydration to check for signs of a dangerous salt-wasting crisis ^[12][11].
• Second-Tier Testing (LC-MS/MS): Some screening programs automatically run an advanced follow-up test on the original blood spot called LC-MS/MS (liquid chromatography-tandem mass spectrometry) ^[8]. This test is more specific: it measures 17-OHP alongside other steroids (like 21-deoxycortisol) to help distinguish between a stressed premature adrenal gland and true CAH, sharply reducing the number of false-positive alarms ^[13]. However, this specialized test is not available everywhere and its results still require careful clinical interpretation.
• Repeat Newborn Screening: For certain low-risk or borderline results, the protocol may require a repeat heel-prick screen a few days or weeks later, depending on local guidelines ^[10][14].

If true CAH is confirmed, highly effective daily hormone replacement medications are available. Your medical team will teach you exactly how to manage the condition and keep your baby safe during illnesses ^[15].