The performance data that can be found here are most often sensitivity, specificity and the negative predictive value NPV. As D-dimer assays are used as an aid in diagnosis to rule out venous thromboembolism VTE — i. However, the prevalence of disease has an influence on the negative predictive value, which tells us the percent of all with a negative test that are not ill. Therefore it would be an advantage if negative predictive values were presented with information about the specific prevalence they were calculated for, i. Figure 1 shows how NPV is influenced by the prevalence of disease and in addition it shows the influence that assay sensitivity also has.
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Published by Noah Berland on February 28, February 28, In our last two pieces on pulmonary embolism PE , 1 and 2 , we discussed five studies that attempted to describe the prevalence of pulmonary embolism in patients presenting to the ED with syncope.
In this post, we will review some common methodologies for working up PE in the ED. PE is one of those usually considered not-to-be-missed diagnoses. It has traditionally been thought that a missed PE can be devastating, although recent studies have started to challenge this idea.
How does one work up a patient with a suspected pulmonary embolism? The Wells criteria are used to objectively generate a pre-test risk for a particular patient. Clinical signs and symptoms of DVT: 3 pts 2. PE is 1 diagnosis OR equally likely: 3 pts 3. Immobilization for at least 3 days OR surgery in the previous 4 weeks: 1.
Hemoptysis: 1 pt 7. As such, it is generally not recommended to use a d-dimer in high-risk individuals. Another way to look at this is that the high-risk population has such a high prevalence that the negative predictive value NPV of d-dimer testing is lowered, making the d-dimer test unsuitable for ruling out PE. If you remember back to your biostatistics, NPV is changed by prevalence, where specificity and sensitivity are not. To reduce the number of tests and d-dimers ordered, many clinicians then apply the PERC PE Rule-out Criteria to generate a more acceptable post-test probability.
No recent surgery or trauma within prior 4wk 6. No hemoptysis 7. No estrogen use 8. For the experienced clinician who can reliably assign a pre-test probability of PE, PERC can be used without a Wells score to determine a pre-test risk.
What may be considered the most commonly accepted way to improve the pLR of d-dimer tests and decrease inappropriate testing is the age-adjusted d-dimer. The nLR of the test is 0. As patients get older, age-adjusted d-dimer loses some of its sensitivity but has improved specificity. Figure 3. The negative likelihood ratio is 0. Figure 4.
They broke patients into three levels of risk using Wells: low risk , moderate risk 4. It should be noted that although they had no false negatives, they lost 9 patients to follow-up in their low-risk group.
I performed a worst-case scenario analysis assuming all 9 of those patients had a PE to generate a nLR of 0. Figure 5. PEGED study. Of note nLR is calculated based on worst-case analysis where all 9 patients lost to follow up had PE. In their study, no PEs were missed based on any rule-out criteria. This is what many labs use to measure d-dimers, however, there is another unit that you need to be aware of, DDU, which stands for d-dimer Units.
So which algorithm should you use? YEARS algorithm is validated and may even work in pregnant patients. Personally, I use a mixture. I use Wells, and then for the very low-risk Of note, this methodology is not itself validated, but I believe supported by the evidence. References: 1. Kline JA, Kabrhel C. J Emerg Med [Internet] ;48 6 — Ann Intern Med. Prospective validation of wells criteria in the evaluation of patients with suspected pulmonary embolism, Wolf, Stephen J.
Journal of Thrombosis and Haemostasis, 6: Journal of Thrombosis and Haemostasis, Wolf, S. Annals of Emergency Medicine, 71 5 , pp. N Engl J Med [Internet] ; 12 — New England Journal of Medicine. The following two tabs change content below.
Published by Noah Berland on February 28, February 28, In our last two pieces on pulmonary embolism PE , 1 and 2 , we discussed five studies that attempted to describe the prevalence of pulmonary embolism in patients presenting to the ED with syncope. In this post, we will review some common methodologies for working up PE in the ED. PE is one of those usually considered not-to-be-missed diagnoses. It has traditionally been thought that a missed PE can be devastating, although recent studies have started to challenge this idea.
nomogrammer: Fagan’s nomogram using ggplot2
Taujinn View March 9, LRs are generated from the sensitivity and specificity of a given test as we can see:. This nomogram is designed in three parallel longitudinal axes: In the case of medicine, a radiography or CT with contrast medium is more expensive and carries a higher risk for the patient than an ultrasound for example. We also have to invert the scale for the log pre-test odds. The likelihood ratio for a negative result is 0.
Comparing D-dimer assays using likelihood ratios and Fagan nomograms