Firstly, the red cell count and other measurements will show whether the person is anaemic - defined as having less than the normal amount of haemoglobin. Symptoms of anaemia are unusual until the haemoglobin has fallen below 10g per 100ml. Anaemia of this severity is usually obvious enough from features such as pallor and lack of energy, but the blood count and the smear will refine the diagnosis, often pointing clearly to a cause such as recent blood loss, a vitamin or iron deficiency, or even some more serious disorder such as leukaemia.
The white cell count is also often no more than a confirmation of the working diagnosis of an infectious illness such as pneumonia, shown by an increase in the white cell numbers and of one kind, granulocytes, in particular. An increase in another kind of white cell, mononuclear cells, will suggest the possibility of some kinds of virus infection, tuberculosis, some tropical diseases, and exotic infections such as Rocky Mountain spotted fever.
An increase in eosinophilic white cells, which contain granules which stain a red colour in standard tests, occurs in many kinds of parasitic infections (such as worms in the intestines) as well as in allergic disorders such as hay fever and some kinds of asthma. It may also act as an alert to the possibility of an allergic reaction to a drug such as aspirin. Too few white cells may be a sinister sign, though some entirely healthy people go through their lives with only one-fifth of the so-called normal numbers in their blood. A really low white cell count indicates that the body's immune sytem is faltering. This may be due to damage to the cell factory in the bone marrow from poisonous chemicals, reactions to prescribed medicines, or replacement of the bone marrow by cancer cells. It is also a feature of some infectious illnesses such as typhoid and glandular fevers, malaria and Aids.
Examination of the blood smear is important in the diagnosis of anaemia and white cell disorders; the proportion of young cells, for example, will indicate how well the body is responding to the illness. The microscopic examination may also come up with surprises such as an unsuspected infection.
If the blood counts and smear are normal then a serious illness seems less likely, though it is not ruled out. We are still many years away from the science-fiction fantasy of a machine that takes a blood sample and, within a few seconds, produces a complete and reliable diagnosis.