The description below is of historical interest only as since September 2004 this test is no longer offered. It has been supplanted by the rt-PCR test methodology.

This test takes advantage of the observation that in acute CMV peripheral blood lymphocytes are one of the "factories" where CMV is manufactured. Cells producing the virus can be stained with fluorescent antibody to detect the presence of CMV antigen. In order to detect this, whole cells need to be harvested, in this case the Buffy coat is used because this is where infected cells are found. The problem is, that once blood is collected for diagnosis, the white cells, which include lymphocytes, begin to lyse and after a few hours have passed there are no longer sufficient whole cells present to perform the test. The critical step in this test is therefore the time from collection until the blood is received in the Virus Laboratory at St Joseph's Health Center. A delay of more than six hours makes the test inaccurate as too few cells survive to give a representative sample.

When the sample is received it is spun down, the Buffy coat is removed and a cell count is performed to ensure there are sufficient cells present. Smears containing 100,000 Polymorphonuclear cells (performed in duplicate) are made and stained with fluorescent antibody (positive and negative controls are also included). The smear is then searched for fluorescing cells and if present the test is reported as positive. Although the test is not approved for semi quantitative purposes many laboratories, including ours, use it in this fashion as a crude measure of viral load. In this case the numbers of cells fluorescing per smear are indicated and on repeated testing some estimate can be made about the efficacy of therapeutic interventions. For instance, if the initial numbers of cells fluorescing are 20 per smear and after a few days therapy of gangcyclovir a repeat test shows 5 cells per smear that are positive then it can be assumed that the therapy is having the appropriate effect. It is of course necessary to have about the same cell count in both samples at the time the test is done or comparisons are meaningless.

To improve the clinical value of this test the following points should be remembered.

1. Phone the lab and discuss the patient to see if it is the best test to use.
2. Determine when to take the sample.
3. Use the appropriate packaging to ensure a prompt delivery to the viral laboratory.

This test is particularly useful in situations were serology unreliable e.g. patients who have had recent blood transfusions or are immunosuppressed, particularly patients
post transplant. It is not as useful in conditions where white cells are limited to start with. All antigen test requests are set up for CMV shell vial culture so the antigen
test result is followed by a culture report within a day or so which may provide supportive information for the antigen test result.