Prenatal testing Flashcards
What is prenatal testing?
Diagnostics before birth. The determination of whether a fetus has a disease or disorder while still in the womb. Implies analysis of the fetal genome (by various techniques).
What are the two main types of prenatal testing?
Prenatal screening and prenatal diagnosis.
What are the differences between prenatal screening and prenatal diagnosis?
Prenatal screening are performed on large populations. They’re inexpensive, low risk, and non-invasive methods.
Prenatal diagnosis are performed on selected groups. They’re generally more costly, higher in risk and imply invasive methods.
What is sequential screening?
Provision of an initial set of tests followed by another set usually for those at low or intermediate risk.
What is contingency screening?
Provision of an initial set of tests followed by another set only for those initially found to be at intermediate risk.
What is meant by “secondary” screening?
A follow-up screening test for those women who were positive by an initial primary test.
Name some of what characterizes a screening.
The condition tested for should be considered an important health problem.
The natural history of the condition should be understood.
There should be accepted management for affected cases.
Facilities for diagnosis and treatment should be available.
The screening test needs to have acceptable detection rates and false positive rates.
The test should be acceptable to the population.
The test should be cost effective.
There should be an agreement policy on who to screen.
What is detection rate?
The same as sensitivity. It is the proportion of affected individuals with positive results.
Sensitivity = (True positives) / (Total number of affected individuals).
(Total number of affected individuals = False negatives + True positives.)
What is false positive rate?
The ratio between the number of false positives and the total number of actual negative events. (Total number of actual negative events = True negatives + False positives.)
It is the same as 1 - specificity.
What is positive predictive value (PPV)?
The proportions of positive results that are true positives. PPV = (true positive) / (total number of positives). (Total number of positives = True positives + False positives.)
How are the odds of being affected given a positive result (OAPR) calculated?
The ratio of affected to unaffected cases among individuals with positive results.
OAPR = (True positives) / (False positives)
or
OAPR = 1 / ((False positives) / (True positives))
What is meant by a test’s precision and accuracy?
Precision: The closeness of the measurements to each other. A description of random errors.
Accuracy: The closeness of the measurements to a specific value. A description of systematic errors.
What are the purposes of prenatal testing?
Clarify risk.
Reduce anxiety in high risk situations.
Allow informed decision about pregnancy management to parents.
Enable timely medical or surgical treatment before or after birth.
Prepare parent and healthcare staff for the delivery of the child or for the likelihood of a stillbirth.
Diagnose high risk pregnancies early so that delivery can be scheduled in a tertiary care hospital - optimizing the outcome.
Give the parent a change to prepare psychologically, socially and financially.
Who should we refer for prenatal testing?
Couples who have previously had a child with a:
- Chromosome abnormality
- Severe autosomal dominant or recessive disorder (that can be identified through fetal diagnosis)
- Severe X-linked recessive disorder
- Mitochondrial disorder
- Structural defect (of clinical relevance)
Couples who are carriers of a faulty gene or
chromosomal rearrangement.
Maternal age above 38 years.
Maternal consumption of teratogenic medications.
Overall evaluation (social aspects, maternal
disease, family situation).
What is a genetic aberration?
An abnormality of the chromosome number in a cell.
What is amniocentesis? When can the procedure be done?
An invasive method of prenatal testing. In this procedure, a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amniotic sac surrounding a developing fetus. The procedure is done transabdominally and is guided by ultrasound.
The procedure can be done from the 15th week of pregnancy.
What is chorionic villus sampling? When can the procedure be done?
A form of prenatal diagnosis to determine chromosomal or genetic disorders in the fetus. It entails sampling of the chorionic villus (placental tissue) and testing it for chromosomal abnormalities. It can be performed in a transcervical or transabdominal manner. (Transcervical method is uncommon.) Ultrasound is used to locate fetus and placenta.
The procedure can be done from the 10th week of pregnancy.
What is the risk of miscarriage following amniocentesis?
0,5 %. (Higher in pregnancies with multiple fetuses.)
What are the possible complications following amniocentesis?
Miscarriage. Fetomaternal bleedning (Rh-sensitization).
Which chromosomal analysis is commonly used for rapid aneuploidy testing?
Qualitative Fluorescence - Polymerase Chain Reaction (QF-RCR).
What are sources of errors when conduction chromosomal analysis from fetal DNA sampled through amniocentesis or chorionic villus sampling?
Maternal contamination.
Mosaicism.
Ambiguous findings (supernumery marker chromosomes).
Besides amniocentesis and chorionic villus sampling, what are other methods of invasive prenatal testing?
Cordocentesis (blood sample from the umbilical cord).
Fetoscopy.
Fetal skin or liver biopsy.
What is preimplantation genetic testing?
A technique used to identify genetic defects in embryos created through in vitro fertilization (IVF) before pregnancy.
What is preimplantation genetic diagnosis?
Testing performed when one or both genetic parents has a known genetic abnormality and testing is performed on an embryo to determine if it also carries a genetic abnormality.
