Too many people believe medical decision-making is easy, and too many people believe it is based on science.
In reality, medicine is more like a Virginia pond in winter. Winters in Virginia generally allow some ice to form, but it is typically so thin that one would never walk on it, for fear of falling through.
Medicine is similar because physicians walk on a thin layer of science overlying a deep pool of ignorance. Even moderate weights will break through the science layer, dumping the physician underwater, trying not to flail.
A simple experiment shows that dunking your doctor is easy. Pretend you have an abnormal blood test and pretend you are taking a medication. Ask your doctor if the medication affects the blood test. The doctor will answer with some certainty, but if you ask, “Is the published evidence for that solid?” he will probably stammer. If you are taking a second medication and you ask him if the effect of the two medicines together has ever been studied in relation to the lab test, he will clench his jaw and stare daggers at you. Forget about three medications.
All physicians want solid, thick science under their feet, as a Minnesota pond would provide. But they’re in Virginia, where conditions simply don’t allow that: humans are so variable in their genetics and habits (and medications), that there is just no way to run experiments for all the permutations and combinations. This situation will worsen as genetic testing proliferates.
When applicable science is unavailable – the usual case – physicians must make decisions based on personal experience and on extrapolation from inapplicable science. This is called “clinical judgment.” It is incredibly demanding mindwork, and it is the reason we must have the smartest, most dedicated people in medicine – they are the ones who can learn to swim.