Triple test

Overview
The triple test, also called triple screen, the Kettering test or the Bart's test, is an investigation performed during pregnancy (usually the second trimester).

Conditions detected
The most common abnormality the test can detect is fetal trisomy 21 (Down syndrome).

In addition to Down syndrome, the triple and quadruple tests preferentially identify fetal trisomy 18, Turner syndrome, triploidy, trisomy 16 mosaicism, fetal death, Smith-Lemli-Opitz syndrome, and steroid sulfatase deficiency.

Values measured
The triple test measures the following three levels in the maternal serum:


 * alpha-fetoprotein (AFP)
 * human chorionic gonadotropin (hCG)
 * unconjugated estriol (UE3)

Interpretation
Low values for AFP and UE3 and high values for hCG and nuchal thickness suggest an increased risk of developing Down's Syndrome. High levels for AFP indicate possible neural tube defects like spina bifida. An estimated risk is calculated and adjusted for the expectant mother's age, weight and ethnicity; if she's diabetic; if she's having twins or other multiples and the gestational age of the fetus. Many of these factors affect the levels of the substances being measured and the interpretation of the results.

The test is for screening, not for diagnosis, and does not have nearly the same predictive power of amniocentesis or chorionic villus sampling. However, the screening test carries a much lower risk to the fetus, and in conjunction with the age-related risk by the patient is useful to indicate the need to proceed to the more invasive tests.

Variations
Only two of the hormones above can be tested for. Then the test is instead called a double test. In contrast, a quad test focuses on yet another hormone in addition. Furthermore, the triple test may be combined with an ultrasound measurement of nuchal translucency.

Double test
Only AFP and hCG are measured. However, the maternal age, weight, ethnicity etc. are still included. A double test is almost as effective as a triple test, because unconjugated estriol, the omitted hormone, is, in practice, not detected at a higher rate in people who have it, than in people without.

Quad test
The term "quadruple test" or "quad test" is sometimes used to refer to the above three tests plus a test for dimeric inhibin A (DIA).