The good and bad of screening tests.
PREVENTION is better than cure. This is an oft repeated advice from well-meaning friends. Yes, getting ill these days can be an expensive affair.
Some diseases can be prevented if we take the appropriate steps − immunisation, wearing safety helmets and practising a healthy lifestyle are good examples. This kind of prevention is also called “primary prevention”. These methods of preventing the occurrence of disease in the first place are the best form of disease prevention.
What is screening?
For many of diseases, health professionals are still in the dark about the actual causes or the best ways of preventing them. Cancer is a good example. Many causes of cancer have been identified, but the best ways of preventing them from happening is still elusive.
Since we cannot prevent them from happening, is it possible to detect the cancer at an early stage when treatment is likely to be more effective? It does sound like a good idea if we can do that. In medical parlance, this is also a kind of prevention – we call this “secondary prevention”, or screening.
Screening aims to try and detect the disease at such an early stage before it causes symptoms or ill health. For example, using mammography (a special kind of breast X-ray), it may be possible to pick up a tiny cancerous breast lump that is not even detectable by the human hand. It is believed that treating the early breast cancer at this stage is easier and more effective (ie the cancer may be curable at this stage).
In the case of breast cancer, the screening test mentioned is mammography (an X-ray). In other cases, the screening test may be a clinical examination by a doctor, a blood test, or a clinical procedure.
The Pap smear is a clinical procedure that samples cells from the cervix of women with the aim of detecting cells that may indicate an increased risk of cancer of the cervix. By detecting the abnormal cells in the cervix (she does not have cancer yet), the affected woman can be treated, thus preventing her from getting cancer of the cervix.
Sometimes, screening can be a simple clinical examination by doctors or other healthcare practitioners.
Did you know that getting your blood pressure checked is also considered screening? High blood pressure hardly ever causes symptoms. If your blood pressure is consistently high (after several measurements), modifying your lifestyle or taking blood pressure pills will reduce your chances of getting bad outcomes such as heart attacks and strokes.
Screening can indeed be good for the health of the individual when the screening tests lead to the detection of serious health problems in the early stages and starting specific treatment to prevent the associated complications.
Since screening is capable of early detection of cancer and other dreadful diseases, is it always a good idea to go for screening? Well, the answer is a guarded yes. It all depends on the type of screening test and the specific diseases that the screening tests are aimed for.
A woman had a blood test called tumour marker CA19.9, which has been touted as a good screening test for cancer of the pancreas. Her CA19.9 level was slightly elevated. Despite extensive investigations such as CT scan of the abdomen and an invasive X-ray called ERCP, no cancer of the pancreas was found. She ended up spending lots of money and had to be subjected to tests that may be harmful to the body.
A smoker requests his doctor to do a chest X-ray to look for lung cancer. If his chest X-ray is normal, does it mean that he does not have lung cancer? Maybe not, as a chest X-ray can only pick up fairly advanced lung cancer, and it is not good enough to detect very early tumour.
The above two case scenarios point to some problems with screening tests: positive results do not always mean the person has the disease (this is called “false positive”), and negative results do not always mean the person does not have the disease (this is called “false negative”).
What it means is that many screening tests that are in common use are actually not good enough. This problem has serious implications for the person requesting for screening because doctors may be forced to initiate a wild goose chase for a non-existing cancer.
And worse still, the person may be wrongly reassured because the screening test is “normal”.
Breast self-examination has been promoted as a good way to detect breast lumps that may be cancerous.
However, studies in various parts of the world have shown that many women are unable to detect small breast lumps despite being taught the technique of breast self-examination.
Women who perform breast self-examination regularly are more likely to have a breast biopsy. Even if breast cancer is detected initially by self-examination, at the end, they did not really fare much better than women with breast cancer who did not do self-examination regularly.
Thus, in developed countries, breast self-examination is now not recommended since it does not do much good and may possible lead to potential harm (such as increasing the chances of needless breast biopsy and the associated anxiety).
What screening should we go for?
Despite the problem with screening tests, there is still a place for screening. The United States Preventive Services Task Force publishes recommendations on screening tests regularly.
In this report, the medical problems, type of screening tests, and the screening frequency are clearly identified. Some examples of useful screening tests are:
·All adults should have blood pressure checks.
·Adults with raised blood pressure should be checked for diabetes.
·Adults 35 years and above should have blood lipids checked regularly.
·Men and women 50 years and above should be checked for colon cancer.
·Sexually active women should have regular Pap smears.
·Women 50 years and above should have mammography once in two years.
·Pregnant women should have a HIV test.
As the frequency of medical problems vary from place to place, and may be altered by personal characteristics, do discuss whether you need screening (and for which disease) with your doctor. You doctor may recommend screening at a younger age for specific conditions if your family has a hereditary type of disease.
If you have certain risk factors (eg obesity, family history of heart attack), you may benefit from earlier checks for diabetes and blood lipids level.
Many of us have friends and relatives who have gone for “screening packages” that include a battery of tests. Not unusually, one or more of these tests may be abnormal and eventually may turn out to be erroneous or harmless. The tumour markers in particular is something that is seldom needed in a general medical checkup.
Screening tests should ideally be done after careful consideration of its benefits and potential harms.