| Triple test | |
|---|---|
| Synonyms | Triple screen,Bart's test |
| Test of | chromosomal abnormalities |
Thetriple test, also calledtriple screen, theKettering test or theBart's test, is an investigation performed duringpregnancy in the second trimester to classify a patient as either high-risk or low-risk for chromosomal abnormalities (and neural tube defects).
The term "multiple-marker screening test" is sometimes used instead.[1][2] This term can encompass the "double test" and "quadruple test" (described below).
The Triple screen measuresserum levels ofAFP,estriol, andbeta-hCG, with a 70%sensitivity and 5%false-positive rate. It is complemented in some regions of the United States, as theQuad screen (adding inhibin A to the panel, resulting in an 81% sensitivity and 5% false-positive rate for detectingDown syndrome when taken at 15–18 weeks ofgestational age)[3] and otherprenatal diagnosis techniques, although it remains widely used in Canada[4] and other countries. A positive screen indicates an increased risk of chromosomal abnormalities (and neural tube defects), and such patients are then referred for more sensitive and specific procedures to receive a definitive diagnosis, often prenatal diagnosis viaamniocentesis, although the stronger screening option ofcell-free fetal DNA screening (also popularly known as noninvasive prenatal 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.[5]
While the triple test can be performed at any point between 15 and 21.9 weeks of gestation, the highest detection rate for open neural defects is given by a test performed between 16 and 18 weeks of gestation.[6]
The most common abnormality the test can screen istrisomy 21 (Down syndrome). In addition to Down syndrome, the triple and quadruple screens assess risk for fetaltrisomy 18 also known asEdwards syndrome, openneural tube defects, and may also detect an increased risk ofTurner syndrome,triploidy,trisomy 16 mosaicism,fetal death,Smith–Lemli–Opitz syndrome, andsteroid sulfatase deficiency.[7]
The triple test measures the following three levels in the maternal serum:[8]
The levels may indicate increased risk for certain conditions or may be benign:
| AFP | UE3 | hCG | Associated conditions |
| low | low | high | Down syndrome |
| low | low | low | trisomy 18 (Edward's syndrome) |
| high | n/a | n/a | neural tube defects (likespina bifida that may have associated increased levels ofacetylcholinesterase in the amnionic fluid),omphalocele,gastroschisis,multiple gestation (liketwins ortriplets), or an underestimation ofgestational age. |
An estimated risk is calculated and adjusted for the expectant mother'sage;[9] if she is diabetic; if she is having twins or other multiples, and the gestational age of the fetus. Weight and ethnicity may also be used in adjustments.[10] Many of these factors affect the levels of the substances being measured and the interpretation of the results:[10]
The test is forscreening, not fordiagnosis,[12] and does not have nearly the same predictive power as that of genetic blood testing (testingcell-free fetal DNA, or invasivegenetic testing which are performed byamniocentesis orchorionic villus sampling. The screening test carries a much lower risk to thefetus than invasive testing, however, and in conjunction with the age-related risk of the patient it is useful to help determine the need for more invasive tests.
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 anultrasound measurement ofnuchal translucency.[citation needed]
Free beta hCG andPAPP-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.[13]
A test of levels of dimericinhibin A (DIA) is sometimes added to the other three tests, under the name "quadruple test."[14] 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.[15]
| Inhibin A | Associated conditions |
| High | Trisomy 21 (Down syndrome) |
| Unchanged | Trisomy 18 (Edwards syndrome) |
| Variable | Trisomy 13 (Patau syndrome) |