First Trimester Screen With Nuchal Translucency (first trimester combined test, or first trimester screening)

Definition

First trimester screening, also called the first trimester combined test, has two steps:

-A blood test to measure levels of two pregnancy-specific substances in the mother’s blood — pregnancy-associated

plasma protein-A (PAPP-A)

human chorionic gonadotropin (HCG)

-An ultrasound exam to measure the size of the clear space in the tissue at the back of the baby’s neck (nuchal translucency).

First trimester screening is done between week 11 to 14 of pregnancy to evaluate your risk of carrying a baby with Down syndrome.

The test also provides information about the risk of trisomy 18.

Blood test

Sample type

Serum

Precautions

-Collect in serum separator tube with gel barrier. Allow blood to clot, avoiding hemolysis. Separate serum from cells by centrifugation.

-Maternal serum specimens must be drawn prior to amniocentesis to avoid contamination with fetal blood.

Interfering factors

-Extra copies of DNA on a mother’s chromosomes may skew some tests, indicating that a fetus has a disorder when the baby is actually healthy.

-Undetected tumors and mosaicism, in which cells within the mother carry a different genetic makeup.

-vanishing twin syndrome, in which one of a set of multiple fetuses is miscarried can cause false positive result.

Gross hmolysis-

-Gross lipemia

Pre-analytical errors

Grossly hemolysis ,lipemic samples,insufficient quantity for analysis,unlabeled or contaminated samples.

The corrective actions

The sample should be rejected and another sample requested.

Post-analytical errors

1-Writing a wrong result or wrong name in the report

2-report were sent to incorrect patient

The corrective actions

1- If the report is not delivered to the patient and this error is discovered, the correct result or the correct name must be written, but if the report is delivered to the patient, you must communicate with the patient, apologise to him, and tell him that an error has occurred and replace it with the correct report.

2-Communicate with patient ,apologizing for the error, and providing him with the correct report.

Reference range

Negative

It is important to remember that positive screening tests are not diagnostic of fetal abnormalities. While they do indicate an increased risk, only a very small number of women with a positive first trimester screen have babies who actually have a chromosome disorder