The triple test, also called triple screen, the Kettering test or the Bart's test, is an investigation performed during pregnancy in the second trimester to classify a patient as either high-risk or low-risk for chromosomal abnormalities. The term "multiple-marker screening test" is sometimes used instead. This term can encompass the "double test" and "quadruple test". The Triple screen measures serum levels of AFP, estriol, and beta-hCG, with a 70% sensitivity and 5% false-positive rate. It is complemented in some regions of the United States, as the Quad screen and other prenatal diagnosis techniques, although it remains widely used in Canada and other countries. A positive screen indicates an increased risk of chromosomal abnormalities, and such patients are then referred for more sensitive and specific procedures to receive a definitive diagnosis, often prenatal diagnosis via amniocentesis, although the stronger screening option of cell-free fetal DNA screening is frequently offered. The Triple test can be understood as an early predecessor to a long line of subsequent technological improvements. In some American states, such as Missouri, Medicaid reimburses only for the Triple test and not other potentially more accurate screening tests, whereas California offers Quad tests to all pregnant women.
The levels may indicate increased risk for certain conditions or may be benign: An estimated risk is calculated and adjusted for the expectant mother's age; if she's diabetic; if she's having twins or other multiples, and the gestational age of the fetus. Weight and ethnicity may also be used in adjustments. Many of these factors affect the levels of the substances being measured and the interpretation of the results:
As maternal weight increases, MSAFP level decreases
African-American women have MSAFP levels that are 10-15% higher than those of Caucasian women for unknown reasons
Having multiple gestations, such as twins, increases MSAFP because each fetus secretes its own AFP
Incorrect estimation of gestational age is the most common cause of abnormal MSAFP levels
The test is for screening, not for diagnosis, and does not have nearly the same predictive power of amniocentesis or chorionic villus sampling. The screening test carries a much lower risk to the fetus, however, and in conjunction with the age-related risk of the patient it is useful to help determine the need for more invasive tests.
Variations
If only two of the hormones above are tested for, then the test is called a double test. A quad test tests an additional hormone, inhibin. Furthermore, the triple test may be combined with an ultrasound measurement of nuchal translucency.
Double test
Free beta hCG and PAPP-A are measured. However, the maternal age, weight, ethnicity etc. are still included. In the UK the double test is part of the combined test for prenatal diagnostics.
Quadruple test
A test of levels of dimeric inhibin A is sometimes added to the other three tests, under the name "quadruple test." Other names used include "quad test", "quad screen", or "tetra screen." Inhibin A will be found high in cases of trisomy 21 and unchanged in cases of trisomy 18.
