A doctor who failed to diagnose a rare disease in a child recently drew flak in local news. Comments emerged divided. A kiasu-kiasi Singaporean like me might say “yes we should send everybody for testing so this doesn’t happen again!”
1. Should we really be testing everyone?
But if every single person were to be tested for diseases they might not be at high risk for, it is going to cost the public healthcare system too much, and might drain resources from actually treating diseases.
Before we agree too quickly with the usual cost-containment narrative, there is another side of the story we ought to look at.
Preventive medicine, although it has “medicine” in its name, is about helping patients NOT have to take medicine! It is about taking steps to identify people at high risk, to prevent a disease from developing or nipping it in its early stages.
Screening and diagnostic testing play a big role in preventive medicine as they allow healthcare providers to assess your risk for a disease and need for further testing and treatment.
2. Unfortunately, screening tests are not fool-proof
Screening tests are by definition, a simple test performed on a large number of people to identify those who have or are likely to develop a specified disease. However, no screening test can tell with 100% accuracy that someone has the disease.
Having a positive screening test result might NOT necessarily mean you have the disease, and vice versa when it returns negative.
To illustrate, let’s pretend that somebody has breast cancer. We can screen for breast cancer using mammography, but the only way to confirm that a person indeed has breast cancer is by doing an invasive breast biopsy.
After a mammography screening test, this person falls in one of the four grey boxes. True positives and true negatives are useful results, because you know the person definitely has or doesn’t have cancer.
The other 2 possible results highlighted in red? Well, not so much.
- A false positive is when she thought she had the cancer, but she doesn’t.
- A false negative is when she thought she was all-clear, but nope, she has cancer.
False negatives (and false positives) are instances when the test does not accurately predict the truth. And these happen for all screening tests – no test is perfect!
3. The perfect screening test
The most ideal screening test would have minimal false negatives (high SENSITIVITY) and minimal false positives (high SPECIFICITY).
Sensitivity and specificity are concepts about the test method itself. It’s kind of like how good the test is at classifying someone as having or not having a disease.
With advancements in science, screening tests are getting closer and closer to predicting the truth, but it's important we keep this in mind when thinking about increasing the use of screening tests.
4. Bonus: Positive and negative predictive values (for the extra geeky)
From a patient’s perspective, I want to know how likely I have the disease, if my test result is positive ie. how likely do I have breast cancer if my mammogram comes back positive.
This brings us to positive (PPV) and negative (NPV) predictive values of a test. Compared to sensitivity and specificity, PPV and NPV depend on the number of people who have the disease – also known as “prevalence”.
Rare diseases like Kawasaki only affect a few people in the population – about 30 cases per 100,000 children under-5 – that’s only about 0.03%! When prevalence is low, the likelihood that you really have the disease will be low (low PPV), even with a test of high sensitivity and specificity.
In normal speak, it means that most of those screened “positive” will eventually be found not to have the disease upon further testing. This factor probably accounted partially for why Dr Chia decided against ordering any tests to rule out Kawasaki disease for her patient - imagine all the unnecessary worry that parents have to suffer if every doctor started ordering these tests for their febrile children.
I think that's probably enough content for an article you're probably just scrolling through on your smartphone. But if you are still intrigued by the science of screening tests (like me), check out this academic review.
I hope this helps to aid understanding of health screening tests, and how over-using them might not be the answer to the problem of under-diagnosing certain conditions.
For Singaporeans, the Screen for Life programme run by HPB has an online tool to find out what screenings are best suited for yourself, friends and family. Information on subsidies are also available.
Sarah is a fully registered pharmacist with the Singapore Pharmacy Council. She’s currently working towards completing a further degree in public health. Things that excite her include a good book, a good cup of coffee and being able to help people use medicines safely.