The key to getting the 'differential' diagnosis right

Recently I had one of those bursting-at-the-seams types of days in the clinic. Every scheduled patient showed up, plus a few extras. Everybody seemed to have burning concerns that needed immediate attention. One patient had newly diagnosed thyroid disease, but the medication was making her feel worse. Another was having strange twinges in his lower abdomen. Another patient said she had no energy and could hardly get through her day.

For each of these presenting symptoms there can be a gamut of possible causes - what doctors call the "differential diagnosis" - that run from the prosaically benign to the concernedly urgent to the immediately life-threatening. The goal is to come up with a broad differential for each symptom, then prioritise them by likelihood and by severity. Testing for every possible diagnosis is not feasible, so the doctor needs to ask the right questions, listen carefully to the answers, do the right kind of physical exam and pay attention to the clinical cues.

If I had the luxury of an hour with each patient, I would carefully sort through every possibility. But the reality is that I, like most doctors, have five to 10 minutes to push the majority of diagnoses to the bottom of the list, come up with the most likely few at the top - being careful to keep in the rare but serious possibilities - and then explain it to the patient.

The ulcer gnawing at the pit of every doctor's stomach is, "What if I miss something serious?" Patients, of course, have a parallel thought process going on. Before they even get to the doctors' office, they've thought about the symptoms that ail them, considered the cause, and probably searched the web for answers. There are now dozens of so-called symptom checkers on the Internet that allow you to enter your symptoms and come up with your own differential diagnosis. But are they accurate?

A group of researchers in Boston set out to find out. Using 45 case vignettes in a medical school curriculum, they entered the presenting symptoms into 23 different online checkers. One-third of the programs got the correct diagnosis on the nose. When looking at the differential diagnosis, 58 percent of the programs had the right ones listed in their top 20 possibilities.

Much of the media coverage of this study focused on how lousy these symptom checkers are. But Ateev Mehrotra, the lead author on the study, made a different observation. "It made me realise just how hard this task is. What we are asking symptom checkers to do is an extremely difficult task," he said.

His words make me reflect on my overwhelming day in clinic. Each patient presented a wide chasm of possibilities that could be nothing, or something, or something horrible. Was the patient with no energy just not getting enough sleep? Or was she anemic, hypothyroid, depressed or suffering from pancreatic cancer?

General practitioners face the biggest challenges because the diagnostic field is so wide open. We all want to get it right, but don't want to over-order tests that can be harmful (and expensive). And we do, on average, get it mostly right.

Doctors' diagnostic accuracy is estimated to be in the range of 80 to 90 per cent. That implies a 10 to 20 percent error rate, but on days when it feels like you are being pelted with diagnostic possibilities and that you have only minutes to make those decisions, 80 to 90 per cent is a comforting percentage.

©2015 The New York Times