When and why

This is a routine blood (haematology) test which can be part of either the general investigation of an acute illness, or regular monitoring of HIV infection or of the side-effects of certain drug treatments (e.g. AZT, cancer chemotherapy).

A small sample of blood is taken from a vein in the arm (venesection or venipuncture). You should ask if you can lie down before the procedure if you feel you think you might faint or become dizzy.

How it will help

A full blood count (FBC), also known as a complete blood count (CBC) can:

• Detect whether someone is anaemic. Anaemia is a shortage of haemoglobin, the substance that allows red blood cells to transport oxygen to the bodys tissues. Normal haemoglobin levels in HIV-negative people are 12-16 grams per decilitre for women and 13.5-18 for men, but are often a little lower in people with HIV. Anaemia can contribute to the symptoms of fatigue and breathlessness. It is more common among people with HIV than in the general population and may be caused by HIV itself, by opportunistic infections or treatments such as AZT.

• Measure the haematocrit, which is the percentage of red blood cells in the total blood volume. It shows the oxygen-carrying capacity of the blood and tells whether the blood is too thick or too thin. The average range is 40-54 percent.

• Measure the number and different types of white cells in the blood. The total number of white blood cells in an average healthy adult is 4,000 to 11,000. If there are insufficient numbers of white cells, especially of neutrophils (neutropenia) then the risk of bacterial and fungal infections is higher. However, this only applies to neutrophil levels below 500; people with HIV often have slightly lower levels of neutrophils than normal, which is not significant. Neutropenia can be a side-effect of treatment with AZT, ganciclovir or cancer chemotherapy. It can be treated by reducing or stopping the dose of these drugs, or with G-CSF which increases the number of neutrophils in the blood by stimulating production in the bone marrow. Eosinophils are white cells involved in allergic-type reactions. The number of these can be elevated during certain parasitic infections. A normal amount is 0-8% of the total white blood cell count. The lymphocyte count is also measured and used to calculate the CD4 lymphocyte count, another important special blood count in monitoring HIV infection (see below).

• Measure the level of platelets. These are small cells essential for normal blood clotting. A shortage of platelets (thrombocytopenia) may result in bruising and bleeding. An excess of platelets can make the blood flow too stodgy. An average count is 150-440, although people with HIV often have fewer than average. As long as the count is not too low, the condition doesnt need treatment. If it gets so low that bleeding occurs, treatment with intravenous gammaglobulin or steroids may be required.