What is the Inhibin A Test Used For?
Inhibin A is primarily used as a second-trimester maternal serum marker for prenatal screening of Down syndrome (trisomy 21), where it is incorporated into the quadruple screen test performed between 15-20 weeks of gestation. 1, 2
Primary Clinical Application: Down Syndrome Screening
The quadruple marker screen (also called "Quad screen") combines inhibin A with alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol (uE3) to assess the risk of fetal Down syndrome. 1, 2, 3
Detection Performance
- The quadruple screen detects approximately 75% of Down syndrome cases in women younger than 35 years and over 80% in women 35 years and older, at a 5% false-positive rate 2, 3
- Adding inhibin A to the triple screen (AFP, hCG, uE3) increases the detection rate by approximately 5-10% compared to the triple screen alone 2, 3
Optimal Testing Window
- The test should be performed between 16-18 weeks gestation (optimal window), with an acceptable range of 15-20 weeks 2
- Accurate gestational age dating by ultrasound is essential for proper interpretation 2
Inhibin A Levels in Down Syndrome
In Down syndrome pregnancies, maternal serum inhibin A levels are approximately 2-fold higher (elevated) compared to chromosomally normal pregnancies during the second trimester. 3, 4
- The median inhibin A level in Down syndrome pregnancies is approximately 2.0 multiples of the median (MoM) 3
- This elevation appears to reflect increased placental production rather than fetal sources 4
Additional Clinical Context
Other Tumor Markers in Ovarian Cancer
While inhibin A is primarily a prenatal screening marker, inhibin may also be used as a tumor marker in certain ovarian cancers (particularly granulosa cell tumors), though this is a separate clinical application from prenatal screening. 1
Important Covariates Affecting Interpretation
Risk calculations must be adjusted for several maternal factors that affect inhibin A levels: 2
- Maternal weight
- Race/ethnicity
- Insulin-dependent diabetes
- Number of fetuses (singleton vs. multiple gestation)
- Fetal gender (female fetuses associated with slightly higher levels) 5
Critical Implementation Points
Laboratory Standardization
- The inhibin A assay is standardized against the First International Standard for Inhibin (Code 91/624) 1
- Results can be reported in pg/mL or IU/mL 1
- Significant variability exists between different assay lots (up to 30%), requiring laboratories to establish and monitor their own gestational age-specific medians 6
Common Pitfalls to Avoid
Technical errors that compromise screening accuracy include: 2
- Failing to use ultrasound dating for gestational age determination
- Not reinterpreting results when revised gestational age dates become available
- Using singleton cutoffs for twin pregnancies
- Inadequate counseling about the screening nature of the test (not diagnostic)
Clinical Management After Abnormal Results
When the quadruple screen indicates increased risk for Down syndrome, the next step is genetic counseling and offering diagnostic testing (amniocentesis) rather than repeating the screening test. 2