Importantly, neither sensitivity nor false-positive rate conveys individual risk. he statistic that patients and providers usually consider to be the test result is the positive-predictive value, which is the proportion of those with a positive screening result who actually have an aneuploid fetus. It may be expressed
TABLE 14-3. Aneuploidy Screening Counseling Elements 1. All pregnant women have 3 options: screening, diagnostic testing, and no screening or testing.
The purpose of a screening test is to provide information, not to dictate a course of action. Diagnostic testing is safe and effective and provides information that screening does not.
2. The diference between a screening test and a diagnostic test. Screening evaluates whether the pregnancy is at increased risk and estimates degree of risk. Detection rate, false-negative rate, and false-positive rate are provided. Cell-free DNA screening does not always provide a result. Irreversible management decisions should not be based on screening test results. With a positive screening result, a diagnostic test is recommended if the patient wants to know whether the fetus is affected.
3. Basic information is provided about each condition covered by the screening test (prevalence, associated abnormalities, prognosis), and screening test limitations.
A benefit of diagnosis includes earlier identification of associated abnormalities.
In case of trisomy 18 or 13, diagnosis may affect pregnancy management if complications arise such as growth restriction or nonreassuring fetal heart rate.
With sex chromosome aneuploidies, phenotypic expression varies widely. Several are so mild they would otherwise not be diagnosed.
4. The patient's a priori risk for fetal aneuploidy may affect her screening test options or election.
Age-related risk information may be found in reference tables.
If a patient has had a prior fetus with autosomal trisomy, robe sonian translocation, or other chromosomal abnorrnality, additional evaluation and counseling are suggested.
Modified with permission from Dashe JS: Aneuploidy screening in pregnancy, Obstet Gynecol. 2016 Jul;128(1 ):181l-194.
as a l:X ratio or as a percentage. he positive-predictive value value reported in a research trial is the proportion of women is directly afected by disease prevalence, so it is much higher with positive screening results who have afected fetuses for women aged 35 years and older than for younger women (see Table 14-4). Negative-predictive value is the proportion of (Table 14-5). Positive-predictive values can also be reported those with a negative screening test result who have unafected for cohorts of pregnancies. For example, the positive-predictive (euploid) fetuses. Because the prevalence of aneuploidy is so
TABLE 14-4. Characteristics of Screening Tests for Trisomy 21 in Singletons
Screening Test Rate Rate Predictive Valuea Quadruple screen:
AFP, hCG, estriol, inhibin 80-82% 5% 3%
First-trimester screen:
NT, hCG, PAPP-A 80-84% 5% 3-4%
Sequential screening:
Stepwise 92% 5.1l% 5%
Contingent 91% 4.5% 5%
Cell-free DNA screening:
Positive result 99% 0.1% Table 14-5 aThe positive-predictive value represents the overall population studied and cannot be applied to any individual patient.
NT = nuchal translucencyf PA.PP-A = pregnancy-associated plasma protein A.
Data from Baer, 2015f Gil, 2015; Malone, 200Sb; j\lorton, 2015; 2014; Quezada, 2015; Dashe, 201l6.