Diagnostic procedures

Screening tests such as combined screening, serum biochemistry, as well as tests of free fetal DNA are not diagnostic. All have false posi­tives and false negatives, and a confirmatory diagnostic procedure is required.

Indications for diagnostic procedures (28) include:

• increased risk of Down syndrome on screening tests (see ‘Screening for chromosomal abnormalities').

• identification of structural abnormalities known to be associated with chromosomal imbalance

• parental (usually maternal) carrier status of chromosomal rearrangement

• increased NT even in the apparent absence of other structural abnormalities

• parental carrier status or affection of single gene disorders or chromosomal microdeletions

• genetic disorders for which molecular genetic diagnosis is available.

Advances in technology have resulted in changes in the indica­tions and choices of diagnostic procedures. The advantage of these techniques is that they can be performed on uncultured amniocytes or trophoblasts. Most centres have discontinued conventional kar­yotyping using light microscopy. The type of tests performed on the sample depends on the indication for sampling:

• Increased risk for commonly seen trisomies—this risk assessment can be on maternal age, or on some form of tests (ultrasound, ma­ternal serum biochemistry, or both) in addition to maternal age. It can also be following a high-risk result for cell-free DNA analysis on maternal plasma. In these cases, most centres will test only for commonly seen trisomies (21, 18, and 13). Sometimes, mono­somy X (Turner syndrome) is also checked for. Usually quanti­tative fluorescent polymerase chain reaction is the technique utilized to check chromosomal copy number.

• Identification of structural abnormalities, suspicion of rare chromosomal abnormalities (other than trisomy 13/18/21), or imbalance of genetic material including (micro-) deletions or translocations—this is usually looked for using chromosomal microarray analysis.

• Single gene disorder or methylation defects—specific genes must be tested to identify a mutation for prenatal identification of single gene disorders.

With all diagnostic procedures there is a small chance of a labora­tory failure. The usual reason for failure to obtain a result by the la­boratory is because more than one cell line is identified, raising the suspicion of maternal contamination or mosaicism, which may or may not be confined to the placenta. The risk of confined placental mosaicism is particularly relevant to the chorionic villus sampling (CVS) procedure.

The following diagnostic tests are available:

Chorionic villus sampling

CVS can be performed transabdominally, or by a transcervical ap­proach. CVS is usually performed after 11 (11⁺⁰) weeks of gesta­tion. The transabdominal route is perhaps the most commonly used method for CVS. This is performed under direct ultrasound guid­ance. The single-needle technique uses a single 20-21-gauge needle or a double-needle technique with a 17-18-gauge outer needle and a 19-20-gauge inner sampling needle. The double-needle technique avoids the use of multiple skin punctures in the event of obtaining an inadequate sample with the first pass. The transvaginal technique uses a specially designed transcervical CVS cannula. A biopsy for­ceps appears to be a safe alternative.

The procedure is associated with a small risk of a miscarriage. Wijnberger and co-workers substantiated a learning curve for CVS and showed that the safety and success of this procedure was strongly related to the number of prior procedures performed by an operator (29).

Amniocentesis

Amniocentesis involves insertion of a thin needle into the amni­otic cavity to aspirate amniotic fluid.

Fetal blood sampling

This procedure involves passing a thin needle into the umbilical cord (usually the placental end) to obtain a sample of the fetal blood. With advances in newer genetic tests, fetal blood sampling is rarely necessary. The procedure is also associated with a small risk of a miscarriage.

A written consent should be obtained before performing a diag­nostic test. Women should be informed about the reason for a diag­nostic test, what tests will be performed, and how long the results would take.

Consent should have the following:

• The national and local risks of procedure-related pregnancy loss.

• The accuracy and limitations of the particular laboratory test(s) being performed, with information about failure rates and estima­tion of when reports will be available.

• How the results will be communicated.

• The reasons for seeking medical advice following the test.

• Anti-D should be administered post procedure if the woman is rhesus D negative.

It is recommended that units and operators should keep a record of frequencies of multiple insertions, failures, bloody taps, and post­procedure losses.