Well, they may not be completely useless. Like any other diagnostic test,
the clinical utility of the test is based upon the performance of the test
AND the characteristics of the patient.
What was needed in this case (and in many other emergenct assessments) is an
estimate of the pretest probability of disease for PE in the patient
described. All the information needed was not given, but by virtue of the
fact that the patient had recent surgery they are AT LEAST moderate risk!
This group of patients cannot have PE excluded on d-dimers alone.
If you want the maths:[an alternative is a nomogram as shown in many of the
ebm books]
1) What's the pretest probability?
If we clinically guestimate (ok this bits a bit woolly for PE, you can
generate good pretest probabilities for things like DVT and chest pain, for
PE we can still make a clinical estimate) that this patients pretest
probability of disease is 30%???
2) What's the likelyhood ratio?
If d-dimers in PE perform at 85% sensitivity and 64% specificity [1] (there
are several papers - I have chosen one for demo- this is for whole blood
assay NOT the ELISA tests which do not perform as well(allegedly)). The
likelihood ratio for a negative result is therefore 0.23. i.e you are about
0.23 as likely to have a PE with a negative test as you were before.
3) Combine the likelihood ratio with the pretest probability
0.3 x 0.23 = 0.069 = 7% So you've reduced the risk, but not to a level at
which I would feel happy to send the patient home (because it's such a
dangerous condition). Further investigation is clearly needed (though none
of those are perfect either!!!).
[You can also do the calcs for a postive result (the LR=2.7), this gives a
post test result of 81% - so a positive result does not even prove the
diagnosis.]
HOWEVER, if your pretest probability was as low as 5%, a negative test would
give you a post test probability of 1%. That's a much better risk than most
of us manage at the moment!! They could go home (remember no medicine is
perfect - we just reduce the risk, 1% would be pretty good) with
instruction. I don't know what others think, but from what I have seen in UK
we just don't investigate the low risk ones at all!
You can make this sort of thing even more useful if you calculate different
LR's depending on how abnormal the test result is, making the tests even
more powerful.
What is needed for PE is a scoring system (like exists for things like dvt
which would allow us to get a good estimate of pretest disease). Diagnostic
tests virtually NEVER fully exclude or prove a diagnosis. They just make it
more or less likely. What you then do depends on 1) what you have changed
the risk to and 2) what the consequences of getting it wrong are.
Simon
NB: I am not proposing that we issue a huge list of LR's and pretest
probabilities to our SHO's (maybe as a PDA???). But this is exactly the kind
of questions we should be asking when we set up
protocols/guidelines/diagnostic strategies etc.
----- Original Message -----
From: Rowley Cottingham <[log in to unmask]>
To: <[log in to unmask]>
Cc: <[log in to unmask]>
Sent: Sunday, June 04, 2000 11:17 PM
Subject: Re: D-dimers in PE
> D-dimers are a complete waste of time, and certainly can not be used to
exclude a
> hypercoagulable state.
>
> Best wishes,
>
>
> Rowley Cottingham
>
> [log in to unmask]
>
[1]
Ginsberg JS. Wells PS. Kearon C. Anderson D. Crowther M. Weitz JI. Bormanis
J. Brill-Edwards P. Turpie AG. MacKinnon B. Gent M. Hirsh J.
Institution
McMaster University, Hamilton, Ontario, Canada.
Title
Sensitivity and specificity of a rapid whole-blood assay for
D-dimer in the diagnosis of pulmonary
embolism [see comments].
Comments
Comment in: ACP J Club 1999 May-Jun;130(3):75
Source
Annals of Internal Medicine. 129(12):1006-11, 1998 Dec 15.
Local Messages
Held at BMA Library
Abstract
BACKGROUND: Patients with suspected pulmonary embolism often have
nondiagnostic lung scans and may present in circumstances where lung
scanning is unavailable. Levels of D-dimer, a fibrin-specific product, are
increased in patients with acute thrombosis; this may simplify the diagnosis
of pulmonary embolism. OBJECTIVE: To determine the sensitivity and
specificity of a whole-blood D-dimer assay in patients with suspected
pulmonary embolism and in subgroups of patients with low pretest probability
of pulmonary embolism or nondiagnostic lung scans. DESIGN: Prospective
cohort. SETTING: Four tertiary care hospitals. PATIENTS: 1177 consecutive
patients with suspected pulmonary embolism. MEASUREMENTS: All patients
underwent an assessment of pretest probability by use of a standardized
clinical model, a D-dimer assay, ventilation-perfusion lung scanning, and
bilateral compression ultrasonography. Patients in whom pulmonary embolism
was not initially diagnosed were followed for 3 months. Accordingly,
patients were categorized as positive or negative for pulmonary embolism.
RESULTS: Of the 1177 patients, 197 (17%) were classified as positive for
pulmonary embolism. Overall, the D-dimer assay showed a sensitivity of 84.8%
and a specificity of 68.4%. In 703 patients (3.4%) with a low pretest
probability of pulmonary embolism, the likelihood ratio associated with a
negative D-dimer test result was 0.27, resulting in a posterior probability
of 1.0% (95% CI, 0.3% to 2.2%). In 698 patients with nondiagnostic lung
scans (previous probability, 7.4%), the likelihood ratio associated with a
negative D-dimer test result was 0.36, resulting in a posterior probability
of 2.8% (CI, 1.4% to 4.8%). CONCLUSIONS: A normal D-dimer test result is
useful in excluding pulmonary embolism in patients with a low pretest
probability of pulmonary embolism or a nondiagnostic lung scan.
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