Are False Positive CAH Newborn Screens Common for Babies?

Yes, false positive newborn screens for classic Congenital Adrenal Hyperplasia (CAH) are very common. While receiving a positive result can be incredibly frightening for parents, it is important to know that a screening test is a signal, not a diagnosis. The initial test simply flags babies who need further evaluation. Depending on the screening program’s methods, many—or even most—positive results turn out to be false positives ^[1][2].

However, because the test can also indicate a serious condition, every positive screen must be evaluated immediately.

Why Do False Positives Happen?

The standard newborn screen checks for classic CAH by measuring a hormone called 17-hydroxyprogesterone (17-OHP). This hormone builds up in the blood when the body lacks the enzyme needed to make cortisol. However, several normal or temporary factors can cause a healthy baby’s 17-OHP levels to be high:

• Prematurity and Low Birth Weight: Babies born early or small naturally have higher 17-OHP levels as part of their normal development ^[3][4].
• Physical Stress or Illness: Physical stress during a difficult delivery, birth asphyxia, or other neonatal illnesses can cause a temporary spike in 17-OHP ^[5][6].
• Testing Methods: The first-round test used by most states, called an immunoassay, sometimes cross-reacts with other harmless fetal hormones, mistakenly reading them as 17-OHP ^[7][8].
• Timing of the Test: A newborn’s 17-OHP levels are naturally high around the time of birth. If the heel-prick blood spot is collected early, it is more likely to trigger a positive result ^[9][10].

What Happens Next?

Even though false positives are common, doctors take every positive screen seriously. Untreated classic CAH can cause a salt-wasting crisis—a rapid loss of sodium and water that leads to dangerous dehydration and low blood pressure ^[9][11]. Because an infant may appear well before a crisis begins, a positive screen must always be followed up promptly ^[11].

To find out for sure whether your baby has CAH or if the screen was a false positive, your medical team will run confirmatory tests. Depending on your baby’s specific screening numbers, this evaluation may involve:

• A more precise second-step blood spot test (often called LC-MS/MS steroid profiling) that accurately separates 17-OHP from other normal fetal hormones ^[7][8].
• Urgent serum bloodwork drawn from a vein. This often includes testing 17-OHP, cortisol, and electrolytes (sodium and potassium) to see exactly how your baby’s adrenal glands are functioning right now ^[12][9]. Genetic testing is sometimes used later to help clarify the diagnosis, but it does not replace these immediate blood tests ^[13].

Follow your doctor’s explicit plan for testing and evaluation. Do not simply wait and monitor at home unless specifically instructed to do so by a pediatric specialist. Completing the follow-up testing promptly helps ensure timely diagnosis and treatment.

When to Seek Urgent Help

If you are waiting for follow-up appointments or confirmatory test results, you must monitor your baby closely. Go to the nearest emergency room immediately and tell them your baby had a positive newborn screen for CAH if you notice any of the following signs of a salt-wasting crisis:

• Repeated vomiting or an inability to keep feeds down
• Unusual sleepiness, lethargy, or unresponsiveness
• Limpness, weakness, or a seizure
• Signs of dehydration, such as significantly fewer wet diapers than normal