Thomas Bayes belonged to Kent county in England, and was known for his religious bent of mind. He wrote two books. One, titled Divine Benevolence , was purely religious. The other was An Introduction to the Doctrine of Fluxion , a treatise on the mathematics of probability. Only in 1763, two years after his death, when British philosopher Richard Price read the essays from this book at the Royal Society in London, was the impact of the concept really understood. The theory said a ‘pretest probability’ determines the post-test result. It is the Bayes Theorem.
“Doctor, I work in Dubai. I have no complaint as such but I want a full-body scan,” announced the overweight young man. His looks indicated he was one of those successful non-resident Indians.
Today’s medical world is dominated by technology. Tests ranging from CT/MRI/PET scans, angiograms, isotope studies, perfusion tests, all involving cutting-edge technology, are available. These let the physician see deep inside the body, access the anatomy and physiology and detect an abnormality much before it progresses. Most of these tests are safe and painless. Of course they are expensive, but that apart, why not do it for everyone who can afford it? This question has been asked in western societies where the cost of therapy is not an individual’s headache but that of the insurance company.
In a detailed analysis last year published in the British Medical Journal , different screening strategies were tried in a society to see how effective they were. Surprisingly, high-tech screening like scans and blood-markers did no better than a general screening strategy. This confirmed the notion that medical investigations have to be goal-directed, not random.
A CT scan of the head may turn out to be normal in case of Alzheimer’s disease or migraine, where a simple clinical history may point to the correct diagnosis. Similarly, an angiogram may be normal in cardiomyopathy (a heart muscle disease), or pericarditis (an ailment of the protective cover of the heart). Simply because it is high-tech it may not pick out a disease.
But a ‘pretest probability’ changes it all. A 75-year-old man with a history of diabetes, high blood pressure and high cholesterol, with a slight discomfort of the jaw on walking uphill, may indicate a block in the coronary artery ( angina pectoris ) with an urgent need for an angiogram. But a 30-year-old woman with chest pain at a localised point recurring at bedtime is likely to have a non-cardiac gastric ailment.
Biostatistics tells us tests with low false negativity are the ones that are used as a screening tool, while those with low false positive are reserved to clinch diagnosis. If you don’t follow the sequence, you are likely to go wrong all the way. That’s what Thomas Bayes theorised, and it holds good today.
The American College of Cardiology has published the appropriateness criteria for tests to be applied in a complaint. A physician is free to decide who needs which test, but in most cases these guidelines need to be followed to prioritise investigations logically and save cost.
The next time your doctor spends time asking you about your complaint or examines you in detail rather than ordering a scan or a lab test, don’t think he is wasting time. Appreciate his knowledge and respect for the 250-year-old theory of a humble monk who taught us the correct way to arrive at an accurate diagnosis faster, and at less cost.
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