PAPP-A prenatal test PAPP-A - Warsaw

See also: Harmony Test, Nifty Test, SANCO Test.

PAPP-A test is performed between 11 and 14 weeks of pregnancy as a screening test mainly for Down syndrome, Edwards syndrome and Patau syndrome in the fetus. It consists of testing a set of biochemical parameters of the mother's blood and ultrasound parameters of the fetus. PAPP-A test is suitable for patients of all ages, as approximately 70% of children with Down syndrome are born to women under 35 years of age.

In Great Britain and many other European countries, PAPP-A test is the standard of care for every pregnant woman. Based on its result, a group of pregnant women with an increased risk of Down syndrome in the fetus is identified, and these women are offered an invasive diagnostic test – trophoblast biopsy or amniocentesis, which can diagnose or exclude fetal disease.

More than a dozen years ago, when there were no tests such as PAPP-A test, amniocentesis was offered to all women whose risk of Down syndrome in the fetus was increased due to their age – usually after 35 years of age. Currently, after performing PAPP-A test, it turns out that in most "older" women, the risk of fetal disease is low enough that it does not justify amniocentesis. In women under 35 years of age, the test enables detection of affected fetuses, which was previously impossible, because by assumption their risk of fetal disease was too low to offer them the risky procedure of amniocentesis.

In summary, currently the decision to perform amniocentesis or to refrain from it does not have to be based solely on the age of the pregnant woman – thanks to PAPP-A test, most (approximately 90%) of affected fetuses can be detected in women without detectable symptoms of fetal disease risk and amniocentesis can be avoided in many women for whom it was previously the only source of information about fetal health. Currently, PAPP-A test is the test with the highest sensitivity for detecting Down syndrome (test sensitivity means the percentage of detected affected fetuses – overall PAPP-A test sensitivity is approximately 90% – meaning that 9 out of 10 affected fetuses are detected. For comparison, overall triple test sensitivity is approximately 60%).

We have a top-class diagnostic system for determinations using fluorimetry of europium and samarium chelates by Wallac group Perkin-Elmer, one of the few suitable for genetic diagnostics. The choice of biochemical determination method is very important, as many methods due to serum protein fluorescence have insufficient accuracy which may cause incorrect assessment of fetal disease risk.

The determination method we use is one of the two most widely used methods in the world, which means that based on the reagents and laboratory equipment we use, hundreds of thousands of prenatal tests have been performed.

Performance principles

Between 11 and 14 weeks of pregnancy, a blood sample is taken. Usually on the same day, a pregnancy ultrasound is performed, which is necessary for accurate assessment of gestational age. Blood tests measure the concentration of two substances that are markers of Down syndrome, Edwards syndrome and Patau syndrome in the fetus.

The test should be supplemented with an ultrasound measurement of the fluid layer within the subcutaneous tissue of the fetal neck – nuchal translucency. Biochemical markers measured in blood are: pregnancy-associated plasma protein A (PAPP-A) and free beta subunit of human chorionic gonadotropin (free beta hCG). The ultrasound marker is nuchal translucency.

In pregnancies complicated by Down syndrome, PAPP-A levels are lowered, while free beta hCG and nuchal translucency are elevated. In pregnancies with Edwards syndrome and Patau syndrome, PAPP-A and beta-hCG concentrations are lowered and nuchal translucency is increased. These parameter values along with the age of the pregnant woman are used to calculate the risk of Down syndrome, Edwards syndrome and Patau syndrome in a given pregnancy.

What does the term "risk" mean?

Risk is a determination of the probability of occurrence of some event. For example, if the risk of Down syndrome in a given pregnancy is 1:100, this means that out of 100 pregnant women with such a test result, 99 fetuses will be healthy and one will have Down syndrome.

What does an abnormal (positive) result mean?

A positive result means presence in the group with increased risk of Down syndrome and further tests will be indicated – generally amniocentesis or trophoblast biopsy. The result is usually considered positive when fetal disease risk exceeds 1:300 (higher risk – e.g., 1:190, lower risk – e.g., 1:500).

Approximately one in 20 women who underwent PAPP-A test will be in the increased risk group. Most women in the increased risk group give birth to healthy children. On average, approximately one in 50 women who obtained a positive test result actually has a fetus with Down syndrome.

Management in case of abnormal result

Statistically, in approximately 5% of women who underwent PAPP-A test, fetal disease risk will exceed the threshold value, usually defined as 1:300. In such a case, amniocentesis should be considered. Of course, the higher the risk value, the higher the probability of child disease.

What does a normal (negative) result mean?

A negative result means that the risk of Down syndrome in the fetus calculated based on the above parameters is low. In such a case, invasive tests are usually not recommended. Although a negative test result means low risk of fetal disease, it does not exclude it 100%. However, such a result means that fetal disease risk is lower than the risk of pregnancy loss due to amniocentesis or other invasive tests.

Does PAPP-A test detect all pregnancies with Down syndrome?

Not all. With correct assessment of gestational age and introduction of correct medians of all parameters, approximately 90% (nine out of ten) of affected fetuses are detected.

Does PAPP-A test detect other chromosomal aberrations?

PAPP-A test detects most cases of Edwards and Patau syndromes, most cases of Turner syndrome. It also detects some other chromosomal abnormalities in the fetus, but unlike amniocentesis, it does not detect 100% of chromosomal aberration cases.

Limitations of PAPP-A test

The test does not detect 100% of Down syndrome cases and other fetal pathologies. An abnormal test result does not mean fetal disease, but means increased risk that should be verified by other methods – trophoblast biopsy or amniocentesis. An abnormal test result may mean other pathology, therefore in case of a normal amniocentesis result, special care for the pregnant woman and possible diagnostics – e.g., ultrasound in the second trimester – is indicated.

Women over 35 years of age have a greater probability of receiving an abnormal result than younger women. This happens because fetal disease risk calculations are made not only based on biochemical and ultrasound data, but also based on fetal disease risk resulting from the age of the pregnant woman.

However, the influence of the pregnant woman's age on the probability of obtaining a false result is smaller for PAPP-A test than for triple test, because parameters tested in PAPP-A test undergo greater deviations from normal in case of fetal disease than parameters tested in triple test – they are more specific and risk assessment in PAPP-A test depends to a greater extent than for triple test on biochemical parameter deviations than on the woman's age.

In addition, in the group of women over 35 years of age, test sensitivity is higher than average sensitivity in the population, which is approximately 85-90%.

Other detected diseases

Combined test (PAPP-A determination, free beta-hCG and nuchal translucency) is a screening test mainly for Down syndrome and some other aneuploidies in the fetus. The Fetal Medicine Foundation from London coordinating the largest scientific project in the field of first trimester pregnancy screening published data on several dozen other genetic syndromes and developmental defects, in which nuchal translucency deviations were described.

These include syndromes: Achondrogenesis type II, VACTERL Association, Werdnig-Hoffmann disease, Thanatophoric dysplasia, GM1 gangliosidosis, Blomstrand type osteochondrodysplasia, Osteogenesis imperfecta type II, Beckwith-Wiedemann syndrome, EEC syndrome, Fetal akinesia deformation sequence (FADS), Fryns syndrome, Hydrolethallus syndrome, Jarcho-Levin syndrome, Joubert syndrome, Meckel-Gruber syndrome, Nance-Sweeney syndrome, Noonan syndrome, Perlman syndrome, Roberts syndrome, Short-rib polydactyly syndrome, Smith-Lemli-Opitz syndrome, Trigonocephaly "C" syndrome, Zellweger syndrome.

Of course, this does not mean that the above diseases can be diagnosed based on the combined test result, but one should think about the possibility of these diseases especially if, as a consequence of an abnormal test result, e.g., amniocentesis is performed and a normal fetal karyotype is found. The role of the geneticist doctor in such a case may be to recommend other tests, e.g., genetic ultrasound, molecular tests, etc.

It is worth emphasizing that the above diseases are not revealed in routine invasive "genetic" tests (amniocentesis, trophoblast biopsy, cordocentesis). Abnormal test results also occur in a significant percentage of pregnancies that will later be complicated by hypertension, preeclampsia, fetal nutrition disorders, gestational diabetes, bleeding – therefore an abnormal test result should prompt the doctor to provide careful care for the pregnant woman in the further course of pregnancy.

Final remarks

PAPP-A test is an alternative to invasive diagnostics (amniocentesis and trophoblast biopsy) in women over 35 years of age. For young women, it provides the possibility of diagnosing Down syndrome and other chromosomal aberrations of the fetus. Compared to triple test, it has approximately 30% higher sensitivity, is performed 4-9 weeks earlier, which is why in Western countries triple test is performed in those pregnant women who reported too late for PAPP-A test.

Limitations of amniocentesis

It should be remembered that amniocentesis is a test mainly for chromosomal aberrations of the fetus – genetically determined diseases are not revealed in it unless we perform molecular DNA tests for additional indications, e.g., family history of a specific disease – which is rare, only possible in families already affected by a genetically determined disease – e.g., cystic fibrosis, hemophilia.

Most developmental defects are not revealed in amniocentesis. Sometimes the amniocentesis result is ambiguous – e.g., a fetal karyotype abnormality may be found, regarding which it is impossible to determine whether it will have phenotypic effects, e.g., intellectual disability. It should be emphasized that amniocentesis is not a test that gives a complete picture of fetal health (even genetically).

Amniocentesis is associated with risk of complications: pregnancy loss (approx. 1%), amniotic fluid leakage, intrauterine infection.

Risk of Down syndrome in the fetus at 12 weeks of pregnancy depending on the age of the pregnant woman.

Age of pregnant woman	Risk of Down syndrome in the fetus
25	1:946
30	1:626
35	1:249
37	1:163
39	1:89
41	1:51
43	1:29

New ultrasound marker of fetal diseases – assessment of fetal nasal bone

It has long been known that children with Down syndrome have abnormalities in nasal bone structure. Assessment of the fetal nose is part of "genetic" ultrasound examination in the second trimester of pregnancy. Recently it has been shown that delayed ossification of the fetal nasal bone is visible in a significant percentage of cases with chromosomal aberrations already between 11 and 14 weeks of pregnancy.

It is currently known that nasal bone assessment increases the sensitivity and specificity of the combined test (PAPP-A). Currently, criteria for nasal bone assessment in ultrasound imaging are already known and data confirming the usefulness of this parameter in assessing the risk of Down syndrome in the fetus. Delayed nasal bone ossification in this period of pregnancy is also characteristic of fragile X syndrome – one of the most common causes of intellectual disability.

Unfortunately, nasal bone assessment is difficult and requires more experience than, e.g., nuchal translucency assessment, among other reasons because the echo of nasal skin can be confused with the echo of nasal bone. Additional assessment of fetal nasal bone gives an increase in PAPP-A test sensitivity greater than performing triple test at a later stage of pregnancy.

Is it purposeful to delay screening diagnostics of Down syndrome until the second trimester of pregnancy?

Delaying diagnostics for chromosomal aberrations of the fetus until the second trimester of pregnancy involves giving up very important markers of fetal disease: nuchal translucency, PAPP-A protein and nasal bone and significantly reduces the sensitivity of any diagnostics. In addition, delay in information about the child's health is perceived unfavorably by pregnant women.

Although the usefulness of nasal bone assessment in Down syndrome diagnostics in the first trimester requires further research, current reports suggest high usefulness of this parameter. The approach adopted in our clinic is very cautious: if we find the presence of nasal bone in the fetus, we do not modify the risk (although probably the presence of nasal bone between 11 and 14 weeks of pregnancy causes a 2-3 fold decrease in Down syndrome risk in the fetus). However, if the nasal bone is not visible, we inform the patient about the probable significant increase in risk.

According to Nicolaides et al. and Wald, with maintained specificity of PAPP-A test with nuchal translucency assessment, additional nasal bone assessment may increase test sensitivity from approximately 90% to approximately 95%. In light of the above facts, it can be assumed that nasal bone assessment increases PAPP-A test sensitivity to values exceeding the integrated test, the test is performed several weeks earlier, there is no need for double blood sampling and the cost of the test is significantly reduced.