Clinical Testing Applications
Overview
Clinical testing applications represent the practical, patient-facing side of genetic laboratory science. This chapter covers newborn screening and prenatal testing methods, two areas where genetic testing directly intersects with public health policy and reproductive decision-making. These topics span laboratory knowledge, counseling, ethics, informed consent, and risk communication.
Newborn screening is a population-based public health program that tests all newborns for a panel of treatable conditions. It is unique in genetics because it is one of the few areas where testing occurs without explicit individual consent (operating under implied consent with an opt-out framework in most states). Understanding the conditions screened, the methods used, the concept of a screening vs. diagnostic test, and the follow-up process after a positive screen is essential knowledge for genetic counselors.
Prenatal testing encompasses a broad range of screening and diagnostic options offered during pregnancy, from first-trimester combined screening to cell-free DNA (cfDNA) screening to diagnostic procedures like chorionic villus sampling (CVS) and amniocentesis. Counseling patients through these decisions requires fluency in the timing, detection rates, limitations, and risks of each option.
Key Concepts
- Screening vs. diagnostic testing: understanding the fundamental difference and why positive screens require confirmation
- Sensitivity, specificity, PPV, and NPV: applying these concepts to evaluate newborn screening and prenatal screening performance
- cfDNA screening: high sensitivity for common trisomies but NOT diagnostic; PPV varies significantly by maternal age and condition
- Invasive vs. non-invasive: the risk-benefit tradeoff between cfDNA screening (no procedure risk, lower certainty) and CVS/amniocentesis (small procedure risk, definitive diagnosis)
- State variation in newborn screening panels: the RUSP (Recommended Uniform Screening Panel) as a federal recommendation, with state-level variation in implementation
- Timing of prenatal options: knowing when each screening and diagnostic test is available during pregnancy
Biochemical Genetics
Biochemical diagnostic testing covers the metabolic side of clinical genetics: enzyme assays, substrate/metabolite panels (amino acids, organic acids, acylcarnitines), and the diagnostic algorithms for inborn errors of metabolism. It is closely tied to newborn screening (which is itself biochemical for most analytes) and to the metabolic conditions across the Conditions library.
Identity Testing
Identity testing uses highly polymorphic markers (STRs, SNPs) to establish or exclude biological relationships and to confirm sample identity in the lab. Clinical applications include parentage testing, twin zygosity, sample-mix-up resolution, and chimerism monitoring after bone marrow transplant.
Together, these topics connect the laboratory science covered earlier in this section to real clinical practice and patient care.
Population Screening
Newborn screening is the public health framework for population-based screening of newborns: the conditions on the RUSP, the laboratory methods (tandem mass spectrometry, immunoassays, DNA-based methods, pulse oximetry, point-of-care hearing screening), and the follow-up process when a screen is positive. It is one of the few areas of genetics where testing occurs without explicit individual consent (implied consent with opt-out in most states), which raises distinct ethical questions around dried-blood-spot storage and screening for conditions without immediate treatment.
Predictive Testing
Predictive testing is genetic testing of an asymptomatic person for a variant associated with adult-onset disease (e.g., Huntington disease, BRCA1/2). The pre-test counseling burden is substantial because results carry psychological, familial, and insurance implications that go well beyond the medical management of the condition.
Prenatal Diagnosis
Invasive prenatal testing (amnio/CVS) is the diagnostic counterpart to NIPS and serum screening: CVS typically at 10-13 weeks, amniocentesis at 15+ weeks. Each has a small procedure-related loss risk and yields fetal genetic material adequate for karyotype, CMA, FISH, or molecular testing. Confined placental mosaicism is the principal interpretive caveat for CVS.
Prenatal Overview
Prenatal testing methods is the conceptual overview that ties screening and diagnosis together: first-trimester combined screening (nuchal translucency + serum markers), second-trimester quad screen, integrated and sequential screening, cfDNA screening, and the diagnostic procedures. Ultrasound findings that prompt genetic evaluation, the timing window for each option, and how to counsel through these decisions all live here.
Prenatal Screening
Noninvasive prenatal screening (NIPS / cfDNA) is now the dominant prenatal screen for common aneuploidies. Sensitivity for trisomy 21 is high, but the test is screening rather than diagnostic and PPV varies sharply by maternal age and the specific condition. Pre-test counseling should set expectations about the difference between screening and diagnosis, the meaning of a positive result, and the option to confirm by CVS or amniocentesis.
Reproductive Testing
The reproductive-testing leaves cover the genetic options that surround pregnancy itself. Carrier screening is offered preconception or early in pregnancy to identify couples at risk for autosomal recessive or X-linked conditions; ethnicity-targeted, expanded, and pan-ethnic panels each have their place. Preimplantation genetic testing (PGT) is performed on IVF embryos to select against a known familial variant (PGT-M), aneuploidy (PGT-A), or structural rearrangements (PGT-SR).