D-dimer Cutoffs Are So 2016

May 10, 2017

Short Attention Span Summary

Fun with D-dimer
If you have a lower pretest probability for PE (5%), e.g. by Wells’ Criteria, you can accept a higher D-dimer cutoff (up to 999 ng/mL) and still consider PE ruled out.  If you have a higher pretest probability (15%), you need a lower D-dimer (<500 ng/mL) to consider PE ruled out; “ruled out” meaning < 3% posttest probability, which many consider below the threshold to obtain CTPA.

Example:
41 year old man, no PMH/PSH, with pleuritic chest pain and HR 105.  His D-dimer is 720 ng/mL.  Should we order a CT pulmonary angiogram?

1. Step 1 – Use a validated scoring system.  His Wells score is 1.3 to 4% pretest probability.  Let’s use the higher number = 4%.
2. Step 2 – Order a quantitative D-dimer.  From the table above, a D-dimer of 720 ng/mL has a fitted iLR = 0.24.
3. Step 3 – Calculate post-test probability. I used a calculator to do this.
   Posttest Probability = 1%

Result: The posttest probability is less than 3%, which is below the threshold to obtain CTPA, even with a D-Dimer of 720 ng/mL (usual cutoff is 500 ng/mL).

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p style=”margin-left:40px”>(Note: This is a corrected post thanks to Dr. Alanna Wong!)

Spoon Feed
It looks like we can start thinking of the D-dimer test based on interval likelihood ratios rather than a dichotomous positive or negative, with 500 ng/mL as the cutoff.  EM Lit of Note has some thoughts on this article.

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Abstract

Acad Emerg Med. 2017 Apr 1. doi: 10.1111/acem.13191. [Epub ahead of print]

D-Dimer Interval Likelihood Ratios for Pulmonary Embolism.

Kohn MA1,2, Klok FA3, van Es N4.

Author information:

1Dept. of Epidemiology and Biostatistics, UCSF.

2Dept. of Emergency Medicine, Mills-Peninsula Medical Center.

3Dept. of Medicine – Thrombosis and Hemostasis, Leiden University Medical Center, C-7-14, Albinusdreef 2, 2300RC, Leiden, The Netherlands.

4Dept. of Vascular Medicine, Academic Medical Center, Room F4-136, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.

Abstract

OBJECTIVE:

To estimate D-Dimer interval likelihood ratios (iLRs) for diagnosing pulmonary embolism (PE).

METHODS:

The authors used pooled patient-level data from five PE diagnostic management studies to estimate iLRs for the eight D-Dimer intervals with boundaries 250, 500, 750, 1000, 1500, 2500, and 5000 ng/mL. Logistic regression was used to fit the data so that an interval increase corresponds to increasing the likelihood ratio by a constant factor.

RESULTS:

The iLR for the D-Dimer interval 1000-1499 ng/mL was essentially 1.0 (0.98 with 95% CI 0.82-1.18). In the logistic regression model, the constant between-interval factor was 2.0 (95% CI 1.9 to 2.1). Using these iLR estimates, if the pre-D-Dimer probability of PE is 15%, only a D-Dimer less than 500 ng/mL will result in a post-test probability below 3%; if the pre-test probability is 5%, the threshold for a “negative” D-Dimer is 1000 ng/mL.

CONCLUSIONS:

A decision strategy based on these approximate iLRs agrees with several published strategies. This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

PMID: 28370759