|To Screen Or Not To Screen |
By Dr CH Yip
What is screening?
Screening for a disease means that a test, investigation or procedure is done on a person who has no symptoms of any disease in the hope of detecting the disease. This is one of the methods for detecting a disease early where the chance of cure is the highest.
A population-based screening programme is a programme where a test is applied to the susceptible population (the population at risk of that particular disease) in the hope of detecting a cancer before any symptoms occur.
|What does an effective screening programme achieve? |
If a screening programme is effective, the end result that can be achieved is a reduction in the number of deaths due to that disease (which was screened) due to early detection and treatment. In cancer, there are very few tests that have been proven by large scale studies to achieve this objective. Large scale studies means studies that have been performed in large enough number of people to draw a conclusion that can be applied to the general population. Small scale studies on the other hand may only reflect the disease pattern in that small group of people –say a 100 people. Therefore, it would not be right to apply the results of a small scale study and generalise it to the whole population.
|Population based screening programmes – Where, When and How is it useful? |
|Population based screening (i.e. screening to be applied to the general population in a country) is very expensive and before embarking on a population based screening programme for any disease, the following questions have to be considered.|
1. Is the disease important enough to merit a screening programme?
The commonest cancers in Malaysia are the breast in females and the lungs in males. Other common cancers are those of the colon (large intestine) and cervix.
2. Is there a reliable test for the disease?
The only two screening tests which has been extensively tested and found to be reliable are mammography for breast cancer and PAP smear for cervical cancer.
Screening by occult blood in the faeces to look for colon cancer has not been found to be reliable. Studies on screening for a cancer of the colon by sigmodoscopy (where a tube is passed into the large intestine via the anus to look for a growth) has not been conclusive.
Screening by a tumour marker in the blood called PSA (prostate specific antigen) has been approved by the FDA (Food and Drug Administration) in USA for screening for prostate cancer. However for those with raised levels, the chance of prostate cancer is only around 30%. This is considered justified in the USA, because prostate cancer has the highest incidence in USA. But in Malaysia, prostate cancer is only the 6th commonest cancer (according to the National Cancer Registry 2002 report).
3. Does screening make any difference to the mortality rate of the disease?
Large studies on screening mammography in the 1960’s to 1980’s in Western countries have demonstrated an almost 30% reduction in the deaths from breast cancer in most studies, although there are some controversial studies where such a result was not demonstrated. The effect on the death rate was only seen in the group aged 50-74 years. However recent data has shown that there is a decrease in deaths even in the 40-49 year age group.
4. Can we be reassured if the screening test is normal or negative for the disease concerned?
No, because cancers can occur in the interval between screenings. Another important reason is that there are false negatives in every test. This means, a negative result is obtained even though the disease (such as cancer) is present. However, rate of false negatives vary depending on what test and for what disease and also the conditions and quality assurance of the tests being run.
5. What is the tests show we may have a disease (such as cancer) – when in actual fact, we don’t have that disease? Can this actually happen? Why?
False positives (where the test is positive but the person does not have the disease) can also occur. This leads to a lot of anxiety. The person may be subjected to a lot of unnecessary and expensive tests to confirm the presence or absence of the disease. False positives just like false negatives are part and parcel of any test and accepted by medical professionals. Therefore, it is important that your tests are interpreted by a qualified medical professional who can explain the results to you and not result in unnecessary anxiety.
And just like false negatives, the percentage of tests which are false positive vary again from test to test and applied to which disease. The quality assurance of the tests and conditions under which it was done and the quality of interpretation of the test in general all contribute to the accuracy of the test.
Take the example of a laboratory test on a blood sample. Do not forget, laboratory errors can occurs, including mix up of specimens or poor storage conditions resulting in the specimen of blood being no longer in the right state for the test to be run on it.
6. Can we intervene successfully? In other words, if we act on the basis of a positive test for a disease such as cancer, will we make a difference to the life of the person concerned?
In almost every cancer, an early diagnosis leads to a better chance of cure. Every cancer when diagnosed early can be successfully treated.
|Barriers to screening |
One of the most important barriers to screening is education. There is a lack of awareness by patients of the benefits of screening. There is also misinformation on what screening tests are beneficial.
The high cost and low yield of any screening examination is a definite barrier to screening. Even in mammography which is the most extensively studied modality for screening for breast cancer, the pick-up rate in the United Kingdom which has a population based screening programme is only 6-7 per 1000 women screened.
Screening requires a lot of personnel - radiologists, surgeons, pathologists, nurses and radiographers. To maintain good quality control, training of man-power and teamwork is extremely important. Population-based screening in countries where resources are limited is probably not justified.
|What about screening of individuals?|
|Certainly, if a person has a high risk of developing a disease, for example if you have a strong family of breast cancer or colon cancer, then screening for these diseases will help to diagnose the cancer earlier. Tumour markers are generally useless for early detection of cancer as they tend to rise only in late stages.|
|Therefore, what shall we do, To Screen or Not To Screen?|
|So… in the end, if you are perfectly fit and healthy, with no symptoms, and there is no strong family history of any particular cancer, should you subject yourself to a test just to find out if you have cancer or not? |
First of all, when you do decide to be screened, you must find out how reliable the test is. If it is a test with a high rate of false negatives (negative test, even when you have a cancer), you will get a false sense of complacency, even if you have symptoms of a particular cancer.
If it is a test with a high rate of false positives (test that is positive even when you don’t have a disease/cancer), you will go through a period of anxiety, and probably go through more unnecessary and expensive tests before you are proven not to have cancer.
Second thing to consider is whether the cancer you are screening for is common or not. If I were a woman, and the commonest cancers in women are breast cancer and cervical cancer, and knowing that there is a reliable test available, I would choose to be screened for those diseases.
One must remember that if any abnormality is found, one must be prepared to go on to be investigated further with further tests and biopsies. In any test, there are false negative and false positives, and one must accept that no one test is perfect.
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Friday, 22 April 2005