Your doctor wants a blood test and you wonder what for and what it involves. The answers are simple and reassuring.

It can tell us an enormous amount about your health. Why are you feeling tired and out of sorts? Why do you get out of breath when you do the shopping or housework? Why are you passing a lot of urine? Why do you look wan and pale, even though you eat well?

It is quite likely that the results of a blood test will give us the reasons why these things are occurring. It will be no stab in the dark, no guesswork. There will be reliable, accurate results on which we can base a diagnosis, and the right treatment.

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Guesswork has gone out of medical diagnosis in many areas. So you are sent to a pathologist (or the doctor might even do it himself) for a blood sample. Actually, there is not much to it, as far as you are concerned. It is all over within three minutes.

A rubber tourniquet is briefly wrapped around the upper part of your arm, and the sister causes one of the blood vessels at the crease of your elbow to stand out. A moment later she inserts a needle, and some dark crimson blood fills the syringe.

A bandage and some manual pressure for a couple of minutes (to make sure blood does not track out and cause a bruise), and it is all over. Your doctor will have the results in the morning, speak to him about them.

The doctor later studies the computerized print-out on your blood. Very quickly, obvious defects leap out and with a possible further physical check, he can arrive at the diagnosis. Treatment then follows.

Below is some of the information that may be gleaned from the blood test, carried out these days on computerized machines called auto-analyzers. As many as 30 different pieces of data may be measured, assembled and printed, often within a few minutes. To see these machines in operation is staggering.

Blood Count: The machine can accurately produce a full blood count.

The red and white cells and platelets in a given volume are counted and the figures written down.

Too few red cells cause anemia, a condition common in women. Anemia is common if menstrual blood loss, or loss from any cause, is higher than subsequent production rates.

Bleeding from any cause, or inadequate iron in the diet may produce anemia. Simple therapy often rectifies it, or there may be a search for the cause.

White blood cells are the soldiers of the body, and are there to destroy invading germs. The figure rises during infections.

In leukemia, (cancer of the blood) the numbers become overwhelming because of a disturbance of the white cell producing areas of the body. Certain white cells increase in allergic people, such as asthmatics and hay fever victims.

The platelets govern the blood-clotting mechanism. Too few may mean problems or that certain drugs may be having a harmful effect, indicating the need for a medical review and action.

Sugar: The serum or fluid part of the blood contains an enormous number of elements, as blood carries almost everything the body uses.

High sugar levels may indicate that the person is prone to, or has diabetes.

Heart checks: Blood tests can yield an enormous amount of data about the risks to your heart. Two special factors can be measured, cholesterol and triglyceride. These are blood lipids or fats.

The higher the level for these fats the higher the potential risk of a future heart attack. It does not mean you will have a heart attack, but simply that the risk is increased.

Action can be taken to lower the risks: Less animal fats, egg yolk, red meat and crustaceans, will reduce cholesterol. Less refined sugar and flour will lessen triglyceride levels.

If an attack does happen, the "cardiac enzymes" also picked up in the blood screen, will indicate the degree of heart damage. These chemicals, such as SGOT and LDH, are poured into the bloodstream by injured heart muscle.

Uric Acid: Do you have joint pains, and a swollen big toe? An elevated uric acid level will indicate you are probably a potential gout victim, but suitable therapy is readily available.

Potassium: This vital element is essential for normal body functioning, to keep muscles working. It may be reduced in patients on diuretics, which are often used for treating blood pressure. Leg cramps are common early symptoms. Appropriate action is readily available.

Acid phosphatase: This is an enzyme found in high amounts in males with cancer of the prostate and is often the first and only indication that a serious disease has started.

Calcium and phosphorus are essential in normal bone structure. But in certain diseases, such as the parathyroids just near the thyroid gland, they may produce abnormal levels indicating internal disease.

Liver Tests: If the liver is not working properly, it will show anomalies in the levels of the proteins albumin and globulin, or other enzymes such as the alkaline phosphatase, SGPT. These need prompt investigation.

Fortunately most blood screening tests show little to worry about, but they are helpful to pinpoint any trouble spots so that prompt medical action may be taken.

The modern tests are an amazing step forward in technology, a far cry from the day when a technician had to spend half a day or longer performing a battery of tests on a blood sample.

lt is hardly surprising that the sophisticated equipment for modern blood tests costs almost $100,000.