ad. 19. A double, triple, quadruple test – what it is, is it necessary and how to interpret results?
The first examinations towards the Down syndrome were based on marking an AFP hormone in a serum. If the result was lower than expected, it was treated as suspicious. A weak sensitivity of these markers caused adding another parameter called β-HCG. That is how a double test came into existence (in Polish literature commonly mistaken with the Combined Screening Test in which PAPP-A + fβ-HCG are assessed), which, nevertheless, did not increase the sensitivity in a significant way. Next, the assessment of the estriol value was added, and the examination was called the triple test.
The triple test measures serum levels of above-mentioned AFP, estriol, and b-hCG and is best performed between 16 and 18 weeks of gestation. After obtaining quantity values, the values in respect of a population (MoM) are calculated in a lab and on their basis the risk of the Down syndrome and the defects of so-called neural tube is determined. The result is treated as suspicious in terms of the Down syndrome when the values of AFP and/or estriol are below 0.7 MoM, and the value of β-HCG is above 2 MoM. In some laboratories yet another parameter is added – dimeric inhibin A (suspicious if the value exceeds 2 MoM), then the test is called a QUAD screen test.
Despite combining several parameters the sensitivity of a triple test is still weak. The triple test is less effective in comparison to a screening test in the first trimester of pregnancy even if it is based only on the measurement of nuchal translucency and the age of a pregnant woman. It gradually becomes the history of diagnostics. The triple test has a substantially weaker sensitivity than a combined test (age + NT + PAPP-A + fβ-HCG). It is also less sensitive than a second trimester “genetic” sonogram.
In some centres in the world a so-called Integrated Test is carried out, which consists in calculating the risk of the Down syndrome on the basis of the age, nuchal translucency, PAPP-A from the first trimester and β-HCG, AFP and estriol from the second trimester. A patient receives the final result not until the second trimester, unless the parameters of the first trimester are extremely alarming. This delay of the result from an integrated test made the combined test to be the international standard of examination towards the Down syndrome.