# D-dimer Cutoffs Are So 2016

## 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?

Adapted from Table in cited article

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).

(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: