Laboratory tests have many uses in confirming or disproving a diagnosis. The most important are tests on specimens of blood and urine, although other body fluids may be examined. Microscopic examination and chemical tests are used to examine both types of sample. Routine tests take a few days to produce results, but some can be done in a few minutes in urgent cases.
Most tests are carried out in hospital laboratories, after samples are sent there.
Blood tests allow diagnosis of disorders of the blood itself, and of disorders elsewhere in the body. In a haematological examination a stained smear of blood is examined under the microscope. Its constituent cells – red cells, which carry oxygen, white cells, which combat infection, and platelets, which aid clotting -are counted and their appearance examined. The presence of haemoglobin is determined chemically. A low haemoglobin level is indicative of anaemia. Different types of anaemia can then be distinguished by examining the red cells. The white cells are produced in greater numbers during active infection, which may assist diagnosis. Different kinds of white cells, recognizable microscopically, predominate in bacterial and viral infec- tions. Less commonly, a high white cell count may result from leukaemia, a malignant over-production of white cells. Examining a bone-marrow specimen, withdrawn from hip or breastbone with a large needle, confirms the diagnosis and type. Abnormal bleeding may result from a lack of clotting factors, haemophilia being the best-known disorder in which this occurs. Diagnosis involves measuring levels of the clotting factors in blood samples. The concentrations of dissolved oxygen and carbon dioxide gases in arterial blood are measured in the diagnosis of severe respiratory illness. Samples are taken by puncturing an artery, usually at the groin. One general screening test that uses blood samples is the ‘ESR’, or erythrocyte sedimentation rate. This is the rate at which red cells (erythrocytes) sink in a fine, upright, glass tube. It is a simple test and gives the doctor general information as to the presence of a disorder.
Levels of many chemical substances in the blood alter in the presence of a disorder. Sodium, potassium, chloride and bicarbonate levels may be assessed in venous blood, as may waste products of metabolism normally excreted by the kidneys. Levels of blood chemicals may change dangerously with illness, causing dehydration, profuse vomiting or diarrhoea. The blood sugar level is an important factor in assessing the condition of a diabetic; and a high level of certain enzymes may indicate a heart or liver disorder. Blood also contains bile pigments (bilirubin) from the breakdown of haemoglobin, which are normally disposed of by the liver and bile ducts. In jaundice the level of bilirubin rises, accompanied by yellow discoloration of the patient’s eyes and skin. Measuring bilirubin in blood thus confirms the diagnosis. Yet another group of substances found in the blood are hormones, which may be measured to test for a disorder of, for example, the hormone-producing thyroid gland.
Centrifuging to concentrate a urine sample is necessary before it can be examined under a microscope. If an infection is indicated by the presence of white blood cells, bacterial sensitivity to antibiotics is found by culturing urine in the presence of certain drugs. This helps the doctor to prescribe the most effective antibiotic.
Red blood cells in significant numbers in the urine may indicate an inflammation of the kidney, or a bleeding polyp or tumour within the urinary system. Chemical examination of urine may reveal sugar in the urine. This is always abnormal and diabetes melli-tus, the usual cause, may be screened for in this way. With dangerously high blood sugar levels, substances termed ketones may appear in the urine and give it the smell of acetone. Abnormal amounts of protein are passed in the urine in certain kinds of nephritis; and in obstructive jaundice bile pigments appear in the urine and darken it.
Finally, special chemical tests designed to identify some types of drugs in the blood and urine are especially useful in detecting suspected poisoning and drug overdose.
Other body fluids and products
Cerebrospinal fluid is a clear liquid that bathes the brain and spinal cord. A sample can be taken by lumbar puncture, using a fine needle inserted low in the back between the vertebrae, under a local anaesthetic. Bacteria and other neurological conditions can then be identified by examining the fluid under a microscope.
Sputum coughed from the lungs may be stained in the laboratory and examined microscopically. Bacteria causing infection can be identified, and maligant cells may sometimes be found in cases of lung cancer. The stools may be chemically tested for blood that is often not obvious to the naked eye. This may alert the doctor to the presence of a tumour of the bowel that is causing small amounts of bleeding. Stools may also be examined for organisms causing dysentery and bowel infections.