A diagnosis of SCID is usually made based on a complete medical history and physical examination, in addition to multiple blood tests and chest x rays.[8] However families with known SCID history can go for diagnosis before the symptoms set in.
One of the easiest ways to diagnose this condition is to count the peripheral blood lymphocytes in the child (or those in the cord blood). This is done by two tests; the complete blood count and the manual differential (or a count of the percentage of each different type of white cell in the blood), from which the doctor can calculate the absolute lymphocyte count . There are usually more than 4000 lymphocytes (per cubic millimetre) in normal infant blood in the first year of life, 70% of which are T-cells. T-cells are not present in SCID infants, so the count is low. At an average it is 1500 lymphocytes (per cubic millimetre) [3] .Further if the count is low, the test is repeated and followed by T-cell functioning test if required.
The most definitive test to examine the function of the T-lymphocytes is to place blood lymphocytes in culture tubes, treat them with various stimulants and then incubate them for several days. Normal T-lymphocytes react to these stimulants by undergoing cell division. In contrast, lymphocytes from patients with SCID usually do not react to these stimuli.[3]
Families with history of SCID can also opt for diagnosis in utero of the conceptus(an embryo or fetus with surrounding tissues)[3] .This can only be done if the cause of mutation of the previous SCID affectded infant. Prenatal diagnosis is available via either CVS (chorionic villus sampling) or amniocentesis.
(i) In CVS (chorionic villus sampling) , in the first trimester or the initial 12 weeks of pregnancy, a biopsy of the placenta is performed by using ultrasound technique. Ultrasound technique to utilized to visualize the location of the foetus and also the placenta. The mutation of gene responsible for SCID can be determined from placenta as it is identical to foetus.
(ii) Amniocentesis is a procedure performed under ultrasound guidance where a long thin needle is inserted into the mother’s abdomen, into the uterus, to withdraw small amount of fluid containing infant cells to study. The SCID gene can be studied using cells from the amniotic fluid.[8]
Usually the in utero diagnosis processes involve some risk to the foetus. As a result diagnosis is not preferred as there is a high risk of termination of pregnancy.So early diagnosis of such infants, even before they contract the infections, is to be considered. The first three months of life has a 96% success rate.[3] .
Also the immunoglobulin levels in the body of the SCID infants are low. But since IgG exhibits transplacental movement i.e. it is passed from the mother blood to her foetus’s blood, the infant might have IgG in its blood. So the deficiency of these immunoglobulins cannot be detected unless this IgG is used up in the infant’s body.[3]
