medwireNews: Research from New Zealand shows that the rate of false-positive neonatal screening tests for congenital adrenal hyperplasia (CAH) can be reduced without a loss in sensitivity by assessing additional steroid markers adjusted for birthweight or corrected gestational age (CGA).

Mark de Hora, from Auckland District Health Board, and co-workers explain that liquid chromatography-tandem mass spectrometry (LCMSMS) measurement of 17-hydroxyprogesterone (17-OHP) has been implemented in New Zealand as a second-tier test for CAH, allowing analysis of additional steroids such as androstenedione (A4) and cortisol.

While use of LCMSMS alongside or CGA-adjusted cutoffs for 17-OHP has reduced the number of false-positive results, the team hypothesized that screening for 17-OHP and the steroid ratio of 17-OHP plus A4 divided by cortisol, also with bodyweight- or CGA-adjusted cutoffs, would further improve the test’s positive predictive value (PPV).

The researchers used 177,873 heel prick blood samples that were collected 48–72 hours after birth from 167,672 infants between 2017 and 2020. A second sample was taken at 2 weeks for infants with a birthweight of 1500 g or less, and a third sample taken at 4 weeks from infants with a birthweight of 1000 g or less.

An immunoassay 17-OHP value of 27 nmol/L or above is considered to be out of range and requires second-tier testing with LCMSMS, with the threshold raised to 37 nmol/L in infants with a birthweight of 1500 g or under, they say.

And for the second-tier LCMSMS, a 17-OHP concentration of 23 nmol/L or greater is considered out of range, rising to 34 nmol/L for infants with a birthweight of 1500 g or less.

Overall, 1776 specimens were out of range on the initial immunoassay and referred for second-tier LCMSMS analysis. This led to the diagnosis of CAH in eight patients and no missed CAH cases were reported to the screening programme.

de Hora and colleagues found that infants with CAH had significantly higher levels of 17-OHP, A4 and the calculated ratio compared with healthy individuals but “there was considerable overlap between groups” and the median F concentration was significantly lower in the CAH infants.

The team determined that second-tier LCMSMS cutoffs, based on the upper 97.5th percentile for 17-OHP, were 72, 37, 28 and 22 nmol/L for birthweights of less than 1000 g, 1000–1499 g, 1500–2499 g and 2500 g or above, respectively. For the steroid ratio, the cutoffs were 2.3 for birthweights of less than 2500 g, falling to a cutoff of 1.4 for infants with a birthweight of 2500 g or higher.

And when using CGA for second-tier LCMSMS, the 97.5th percentile cutoffs were 76, 41, 26 and 26 nmol/L for less than 28 weeks, 28–31, 32–36 and 37 weeks or more, respectively. The steroid ratio cutoffs were 2.3, 2.2 and 0.7 for a CGA of less than 28 weeks, 28–36 weeks and 37 weeks or more.

In all, 10 screening samples were out of range for 17-OHP and the steroid ratio using the birthweight or GCA cutoffs, of which eight were considered to be true positives and two false positives.

Screening sensitivity continued to be 100% and screening specificity increased from 99.96% to greater than 99.99% with the adjusted protocols, with a PPV of 80% using either birthweight- or CGA-guided cutoffs.

The researchers note that the true-positive tests were all found in infants with a birthweight above 2500 g and a CGA greater than 36 weeks. The two false positives using the birthweight protocol were reported in the highest birthweight groups, while the two false positives with the CGA protocol occurred in a 28-day-old baby born at 24 weeks and a 2-day-old baby born at 32 weeks.

“The results demonstrate that the use of these second-tier LCMSMS steroid markers would improve the PPV of screening from 11% to 80% if appropriate cutoffs were applied”, de Hora et al write in The Journal of Clinical Endocrinology & Metabolism.

They emphasize that for every four cases of CAH detected by screening, just one false-positive test would occur, translating to a 97% reduction in the previously reported rate.

This will “reduce the impact that repeat bloodspot sample collections have on the resources of health professionals”, they note, but “[m]ost important, the improvements will reduce the anxiety of families caused by additional screening interventions.”

By Lynda Williams

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J Clin Endocrinol Metab 2021; doi:10.1210/clinem/dgab383