Missed and Erroneous Diagnoses Common in Primary Care Visits
bjain at PARTNERS.ORG
Tue Dec 31 15:49:07 UTC 2013
Hi John, I congratulate you on your excellent book which I enjoyed immensely. I found it to be very well written and highly informative. It should be read by every practising physician.
I would like to make a few cautionary remarks about role of probability in clinical diagnosis.
1. A probability, whether prior or posterior symbolises a distribution or frequency in a series of similar patients while clinical diagnosis seeks to determine a disease correctly in a given, individual patient.
2. Therefore, while probabilistic considerations help move the diagnostic process forward, a probability cannot be considered evidence from which a disease is diagnosed in a given patient.
3. Thus, it is entirely reasonable to test for a disease with high prior probability first, as he is drawn from a series of patients in most of whom the disease is present. Therefore, the chance of this disease being found in our patient is high.
4.What would be inappropriate, I suggest, would be to equate high prior probability with high prior evidence and use it as such for diagnosis.
5. This occurs, I suggest in patients with low prior prob. when a disease is ruled out (declared absent) without testing. This has been reported in several cases of acute MI being missed in healthy young women with atypical chest pain.
6. Evidence in a given patient, I believe, is best (and perhaps only) measured by a likelihood ratio. It is customary to diagnose a disease definitively only if a test result with LR of 10 or higher is observed, regardless of prior prob. ( diagnosis of acute MI from acute Q wave and ST elevation EKG changes, LR 13).
7. It is well known from experience, any given disease occurs in different patients with clinical presentations and therefore prior prob. which vary over a wide range. Our goal as clinicians is to diagnose a disease correctly in a given, individual patient regardless of prior prob.
8. In practice, therefore, the correct approach, when confronted by a patient with symptoms, I suggest, is to look upon a presentation as a problem to be resolved and not as evidence for a certain disease.
9. The presentation functions as a clue which makes us suspect several diseases. The order in which we test them may be determinedby prior prob. as discussed earlier.
10.Some test results, at least, such as acute EKG changes (LR 13), positive chest CT angiogram (LR 21), positive venous ultrasound (LR 19) lead to definitive diagnosis of their repective diseases, acute MI, pulmonary embolism, DVT, regardless of prior prob.
11. This suggest, diagnosis of some diseases at least is not performed in a Bayesian manner in actual practice.
12. I believe, it is important to record diagnostic errors in registries as I suggested earlier, so we can study these cases and learn why errors were made. Was a disease not suspected because of its low prior prob. or was an inappropriate test emplyed to diagnose or rule out a disease?
13. Till such registries are formed, we can present and discuss such cases in our forum hwere.
14. I think we need more observational and experimental studies of diagnosis in actual practice to ascertain the method which minimises error.
15. In my view, a strictly Bayesian appproach is not such a method.
Bimal P Jain MD
Northshore Med. Center (Union)
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