What is the medical test

The pros and cons of medical tests (6/7)

Metrics for the quality of a medical test can give you an idea of ​​the benefits and risks of a test.

Positive and negative results can be wrong

Doctors call the result of an examination positive if they have found the signs of the disease they were looking for. A negative finding, on the other hand, says: The doctors couldn't find anything.

No test is one hundred percent certain. Therefore, examinations can produce correct and incorrect findings. Both false positives - then they find the signs of the disease even though you don't have it. Or false negatives: Then the examination does not find the signs, even though you have the disease.

Both mistakes have consequences. In the event of a false negative result, the patient does not receive the necessary therapy. If the finding is false positive, he has to endure further diagnostics and therapies, although it would not be necessary. Sometimes this is the only way to make him sick.

Positive predictive value

How reliable an examination is is shown by its positive predictive value. This value indicates how often the disease is actually present when the test result is positive.

Example mammography screening

For women 50 to 60 years old, the following are known:

  • The probability of developing breast cancer is around 0.8 percent. That means: out of every 1,000 women, eight have breast cancer.
  • If she does have breast cancer, there is a 90 percent chance that the mammogram will tell. That means: out of eight women with breast cancer, seven will have a positive result.
  • Even if she does not have breast cancer, the mammography will give a positive result seven percent of the time. This means that 70 of the remaining 992 women will have a positive mammography, even though they do not have breast cancer.

You can now tell the woman: For every 1,000 examinations, 77 women receive a positive result. That is around eight percent. Only seven of these women actually have breast cancer. That means: ten out of eleven positive results are false; that's about 91 percent.

In other words: the positive predictive value of mammography screening - i.e. the probability that a woman with a positive result actually has breast cancer - is one in eleven, i.e. around nine percent.

Therefore, when you need information about the usefulness of an investigation, it is best to look for information about its positive predictive value.

Parameter "number of necessary treatments"

Early diagnosis examinations are useful when they help to identify a disease early and therefore better treat it. You can see how well they do this, for example, from the "number of treatments required". In the health science literature this value is called NNT - from the English "number needed to treat". It indicates how many people have to take part in an early diagnosis examination, for example, so that one of them can be saved. In general, it states how many people have to be treated or have to take a test so that the desired effect occurs exactly once.

For example, if mammography screening for the early detection of breast cancer saved the life of one in 1,000 women who participated, the NNT = 1,000. 1,000 women must take part in the investigation for one to be saved.