The test is not the disease

When I was a trainee intern we had a patient on my general medical placement present with 2 days of right arm swelling and tenderness, with dilated superficial veins over her arm and upper chest. Her d-dimer was normal. She had an ultrasound of the upper limbs looking for a DVT. This was negative. With a negative d-dimer and USS we were all ready to discharge the patient home (who was otherwise well), however the consultant, a mentor of mine, insisted on a CT venogram. We all rolled our eyes. Eye rolling turned into eye widening as the scan showed the subclavian vein thrombosis we had all been missing.

So what did he have in his armoury that allowed him to be convinced this was a DVT and pursue the diagnosis while the rest of us would have missed it?

An understanding that the test is not the disease. Tests are imperfect and can only shift the probability of disease in one direction or another. But it depends on the starting probability of that disease. What else gives you swelling of the entire arm and dilated venous collaterals of the arm and chest? The starting probability of a DVT is high, and it remains high even after negative initial investigations.

Without going into a topic that other sites have dedicated much more time to explaining better, if the starting pretest probability is high, a negative test is not reassuring. And if the starting pretest probability is low, a positive test does not confirm much.

Take this case which I recall vividly from my house officer years. You review the sodium of an elderly lady on the ward with SIADH. The diagnosis has been made based off a urine sodium of 60. She has been appropriately fluid restricted. Despite this her sodium has fallen further from 120 to 115. Looking through the history you see that she has also had 3 days of diarrhoea and vomiting and poor oral intake. You prescribe gentle fluid rehydration and her sodium starts to improve. This was hypovolemic hyponatremia all along. The pretest probability in an elderly lady with 72 hours of GI fluid losses is high. SIADH, on the other hand, is rare, regardless of the urine sodium.

Or take the man who presents with pulmonary oedema refractory to diuresis who is known to have mitral valve disease. He has a TTE showing only mild MR. But he has a loud pansystolic murmur at the apex that even the medical student can hear. He eventually has a TOE showing severe MR as the cause of his heart failure. The TTE missed the awkwardly pointing jet of MR.

Take one of the very first posts from this blog, about patients who clinically have a small bowel obstruction but have a normal looking abdominal X-ray.

The important lesson to apply from all of these cases when you are seeing patients on ward calls is; it is the patient with the disease, not the test.


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