Clinical Detection Rules Help Diagnose DVT and PE

New clinical detection rules are making it easier for primary care physicians to diagnose deep vein thrombosis and pulmonary embolism. That was the hopeful message from Lee Green, M.D., associate professor of family medicine at the University of Michigan. A recent EPC, or Evidence-Based Practice Center report, outlines the most effective diagnostic schema for VTE yet to emerge from the Agency for Healthcare Research and Quality.

"This is an area that is moving quickly," Dr. Green told the American Academy of Family Physicians Scientific Assembly. "EPCs are based on the best evidence available."

As many physicians expected, the Wells criteria are highly effective at predicting which patients are likely to develop DVT. The criteria routinely identify patients with less than a 10% probability of DVT and select patients who are appropriate for further testing.

Combining a Wells score of less than 2 with a high-sensitivity D-dimer identifies patients who are highly unlikely to have DVT.

"With those two conditions present, you can exclude DVT and end your workup at that point," Dr. Green said. "But there are two caveats."

The first, he said, is an actual calculation of the Wells score. It is not enough to eyeball the criteria and guesstimate a score.

The second caveat is more difficult to overcome. The D-dimer assay must be a true high-sensitivity procedure.

"Not all D-dimer assays are created equal," he cautioned. "There are at least seven different assays on the market and not all are sufficiently sensitive."

The EPC also notes that prediction is more accurate for proximal DVT than for distal blockages.

Detection of pulmonary embolism by Wells score and D-dimer is less accurate. The negative predictive value (NPV) ranges from 72% to 98%.

D-dimer alone is not sufficient to rule out VTE, Dr. Green noted, although it is more accurate in younger patients with proximal disease, no comorbidities and typical symptoms. Again, the problem is lack of standardization in assays and cutoff values.

Ultrasound is both highly sensitive and highly specific for the diagnosis of DVT, but only for symptomatic proximal disease.

"A negative ultrasound means you can very confidently exclude symptomatic DVT above the knee," Dr. Green said. Drop to the calf vein, however, and both sensitivity and specificity drop to about 29%. Sensitivity for asymptomatic disease is 37%.

The biggest change in PE diagnosis is the use of helical CT. For now, there is a wide variation in sensitivity (45% to 100%) and specificity (78% to 100%). Accuracy for emboli distal to the lobar arteries is even lower.

The variation, Dr. Green explained, is due almost entirely to differences in CT scanners currently in use. The University of Michigan and other academic centers are using scanners with as many as 54 detectors. Results fall at the high end of both the sensitivity and specificity range.

Some community imaging centers are using older machines with a single detector and reporting much lower numbers. More detectors are better, he said, but practitioners can expect good results from scanners with at least four detectors.

"It is clear that the multidetector CT will end up as the preferred modality," Dr. Green predicted. "But not all centers use the same type of scanners for all patients. You are still going to have to know what kind of scanner was used on your patient."