Wednesday, July 20, 2005
Two studies give discordant verdicts on value of Wells' DVT prediction rule
Clinical prediction rules when they work best can give physicians an estimate of the likelihood of the disease at issue. The famous Ottawa ankle rules seem to work very well and have decreased the need for ankle x-ray in many patients.Rules regarding the diagnoses of DVT's have generated many articles and much research and lead to several clinical decisions rules the most widely talked about being the Wells' criteria. The criteria can be placed on a PDA and in the best of times an intern with 2 days clinical experience can with the aid of his thumb arrive at the same answer as the seasoned senior chief of the service. However, in regard to DVT diagnosis how often does the intern find himself in the best of times?. According to one of the two conflicting articles in the latest issue of the Annals of Internal Medicine he may be wrong far too often if he relies on the Well's criteria and a normal d-dimer test. The thinking is that if the Wells rules derived probability is low and the d-dimer is normal, then there is no need to do an ultra sound exam. The whole exercise is basically to avoid doing a u/s exam just as the Ottawa ankle rules exist to decrease the number of "unnecessary" ankle xrays. Two articles in the July 19, 2005 issue of the Annals of Internal Medicine give different assessments of the value of the Wells prediction rules. One, from Holland finds there are too many cases of DVT missed. This study involved a number of primary care doctors in Holland seeing real patients in primary care settings. They found 12% of patients with DVT had a low probability Wells score and 3% had a normal d-dimer. The second article, a meta-analysis concludes that if Wells probability is low and the d-dimer is normal, there is no need to do a u/s. The editorialist, Dr. James Douketis, sides with the pro-Wells rule rule article but importantly says the physician should retain the option of overriding a low Wells score by doing venous ultrasonography. He also says the purpose of the Well score is to complement rather than displace the clinician who will use her clinical judgment to fine tune the estimate of disease. I would add -it is more important not to miss a DVT than it is to avoid "avoid" an ultrasound exam. The Wells score was more important when the gold standard test was venography, a procedure that could ironically cause the problem-phlebitis- you were trying to diagnose. I cannot really understand the imperative to "avoid" a test that is the most definitive we have for DVT and is non-invasive, safe and is widely performed and is very sensitive.The last case of DVT I managed was in a 54 year old previously healthy man who had traveled to New Zealand and had a one week history of a tender, swollen calf. I ordered a u/s which showed a popliteal clot and treatment was begun. What would have been gained by wasting time on a d-dimer test? With the various versions of d-dimer of varying sensitivities you are more likely to be mislead by the d-dimer than with an u/s. Why not go with a test that can demonstrate the clot rather than one that might imply a clot might be present?