Diagnosis Recitation

The Dilemma At the conclusion of my “diagnosis” presentation during the recent IAPA meeting, a gentleman from the audience asked what my opinion was regarding the use of D-dimer in the diagnosis of pulmonary embolism. I didn’t have an answer at the time, but needed to develop one!

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Validity Assessment

Was the reference, i.e. “gold” standard measured consistently and) independently, i.e. blind to our target test?

Reference standards results indicating the presence of a pulmonary embolism included: –High-probability ventilation/perfusion scan, –Positive computerized tomographic scan, or –Positive lower extremity imaging.

Acceptable standards for a negative diagnosis were –Normal or very low probability ventilation/perfusion scan or –The absence of a thromboembolic event for 3 months or more

Excluding more than half of the reviewed articles because of quality concerns still resulted in significant differences among the included studies, such as blinding of the radiologist to the D-dimer result (7 of the 11 studies)

So, what do you think? –Was a “gold” standard measured? If not, is what was done reasonable? Why or why not? –What do you make of the “absence of a thromboembolic event” criteria? –Were those charged with “testing the test” blinded?

Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom we would use it in practice)?

Mostly outpatients aged Is this OK given the risk factors? Inherited conditions that cause increased risk for blood clotting Restricted or slow blood flow in a deep vein, due to injury, surgery, or having to stay in bed for a long time Cancer and its treatment Medical conditions such as varicose veins Sitting for a long period of time, such as on long trips in a car or on an airplane During pregnancy and in the 6 week period after delivery Being over age 60 (although DVT can occur at any age) Being overweight or obese Taking birth control or hormone replacement pills Having a medical condition that requires a central venous catheter. A central venous catheter is a tube placed in a vein to allow easy access to your bloodstream for medical treatment.

Was the reference standard ascertained regardless of the diagnostic test result?

Importance Assessment

Sensitivity 95% What does this mean?

Specificity 45% What does this mean?

What is the positive likelihood ratio of this “new” test?

Sensitivity / (1-Specificity) 0.95/0.55 = 1.7 When do we use this?

What is the negative likelihood ratio of this “new” test?

(1-Sensitvity)/ Specificity 0.05/0.45 = 0.1 When do we use this?

Study Recommendation When it is negative, the D-dimer may help rule out PE when the pre- test probability is low! In general, do sensitive tests, when negative, tend to rule things out??

Let’s test this out! Your patient is a totally healthy 30 year old female except for recently completing chemotherapy for a new dx of breast cancer She has had unilateral leg swelling for several days with tenderness to palpation She presents today with sudden onset of shortness of breath and unilateral, pleuritic chest pain

What is the pre-test probability for this patient? What are the possible post-test probabilities if the D-dimer comes back positive (red) and negative (green)?

Finding Pre-Test Probability Well's Scoring System

What if the patient only had the breast cancer? Well's Scoring System