Wednesday, January 11, 2006

Still another algorithm for diagnosis of pulmonary embolism

The Jan. 11, 2006 issue of JAMA has an article (The Chistopher Study,vol..295,no.2,172-179) with a proposed system or algorithm for diagnosing pulmonary emboli (PE) using the Wells decision rule, the d-dimer blood test and the CT scan. It works like this: if the Well's score is less than 4 then a negative dimer sufficiently excludes PE so that no further tests are done. If the Wells number is 4 or more then a CT is needed even if the dimer is negative.Less than 4 is said to make PE Unlikely and 4 or more is Likely.
This was a large (3306 consecutive patients) study from 12 centers in Holland. The absence of PE was not determined by pulmonary angiography but the study relied on a nearly complete 3 month followup. An important issue is which d-dimer test was used. This study used a very sensitive immunosorbent assay (Vidas d-dimer).The immunosorbent and immunoturbidimetric tests are very sensitive-said to be greater than 95%- while the latex agglutination assays are less so. Excluding PE on the basis of a "unlikely"Wells score plus a negative d-dimer will only work well if a very sensitive assay is used.
I wonder if the "dichotomized"version of the Wells decision rule is too simple.All patients in the "unlikely" category are not equally unlikely to have PE. I have a problem with reducing a physician's assessment of how likely the diagnosis may be to a mechanistic rule and apparently excluding any and all other elements that a physician may call upon to decide the likelihood of a diagnosis. for example a decreased o2 saturation. A clinician's "global assessment"( i.e. considering the overall clinical picture not just the check list from Wells) should trump the decision rule.If you believe PE or DVT is a reasonable diagnosis to pursue,testing should be done even if the Wells rule suggests low risk or unlikely and the d-dimer is negative.(show me a blood test that can't be wrong)
An editorial in the same issue is written by a well respected DVT/PE expert ,Dr. R.D.Hull from Calgary who is enthusiastic about the Christopher study authors' proposed algorithm. He says in part " ... firm recommendations can now be made concerning practical and fairly simple diagnostic algorithms for evaluating patients with suspected PE or deep vein thrombosis". D-dimer testing and the increasingly technically impressive CT imaging techniques are giving us better tools for the often elusive and vexing problems associated with venous thromboembolism but I have concerns that reliance on decision rules make things seem more simple than they really are and there is much more to clinical judgment than is captured in a simple checklist decision rule.

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