Collecting samples of body fluids, cells, or tissues for testing
As well as taking your medical history and carrying out a physical examination, your doctor may need to collect a sample, such as blood or urine, for testing, either for diagnosis or to monitor the progress of a disease or the effectiveness of treatment.
Obtaining most types of sample is straightforward and painless, and may be carried out by a doctor or nurse as part of a visit to the surgery or by yourself at home. More complex sampling procedures are performed in hospital. The samples are usually sent to a laboratory for testing, although some tests may be carried out in the doctor’s surgery.
Types of sample
The sample needed and the types of tests carried out depend on the information your doctor needs. Blood and urine samples can be tested to check the function of various organs, and tests on faeces may be used to identify disorders of the digestive tract. Sputum or other body fluids, cells, or tissues may be tested to look for various diseases, such as cancer. The samples most commonly taken by your GP are described here.
Blood tests can help the doctor assess your state of health and look for risk factors associated with certain diseases. For example, a blood sample may be taken to check your blood cholesterol level, which is an indicator of your risk for coronary heart disease. If you visit your doctor with specific symptoms, a blood test may help to diagnose the cause of your illness. Simple tests, such as for blood glucose levels, may be done in the doctor’s surgery. For more complex tests, such as tests for microorganisms, tests for levels of hormones or other chemicals, and tests to look at the blood cells and functions, the sample is sent to a laboratory.
Blood tests usually require only a small sample of blood. In most cases, your doctor or nurse will take a sample from a vein using a hollow needle attached to a syringe. Occasionally, only a drop or two of capillary blood is needed, and this is obtained by using a sterile instrument called a lancet to prick the skin.
Urine tests can provide information about the condition of the urinary tract and about changes in body chemistry associated with disorders of other systems, such as diabetes mellitus. Usually, your doctor will give you a sterile container in which to collect your urine and instructions about how to collect a suitable sample. In most cases, your doctor will carry out simple dipstick tests in the surgery to detect evidence of infection, glucose, or traces of blood or protein. If further tests are needed, the sample will be sent to a laboratory.
Samples of faeces may be used to help diagnose disorders of the digestive system, such as infections, and a faecal sample is used in the faecal occult blood screening test to detect early signs of colorectal cancer. If you are asked to provide a faecal sample, you will be given a special container and instructions on how to collect the sample. In most cases, the sample is sent to a laboratory for testing.
Fluids or secretions from the skin or mucous membranes, such as those lining the throat, are usually collected on a sterile swab (see Having a throat swab taken) to look for evidence of infection. Semen may also be collected for fertility testing. You can collect some samples, such as sputum or semen yourself, and your doctor may give you a sterile container for the purpose. Techniques for collecting other body fluids are covered in the section on your body and disease. Fluid samples are usually sent to a laboratory.
Cell and tissue samples
These types of samples are often used to investigate tumours and may also be used for genetic tests. Cells can sometimes be extracted from body fluids such as urine, or may be scraped from the tissue surfaces of body cavities such as the mouth, throat, or vagina, as in a cervical smear test. Removal of a larger tissue sample is known as a biopsy and is usually carried out in hospital. The various biopsy techniques are covered in the section on your body and disease. Cell and tissue samples are sent to a laboratory for analysis.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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