# The Potential Effect on the Number of Computerized Tomography Pulmonary Angiography if a Clinical Probability-Adjusted D-dimer Is Added to an Age-Adjusted D-dimer

## TL;DR

This study shows that using a higher D-dimer threshold can reduce unnecessary CT scans for diagnosing blood clots in the lungs.

## Contribution

A new D-dimer threshold of 1000 ng/dL is proposed to reduce CTPA use in moderate-risk pulmonary embolism patients.

## Key findings

- Using a D-dimer threshold of 1000 ng/dL avoided 95 CTPA scans in low-moderate risk patients with a 2.1% false negative rate.
- In high-moderate risk patients, 21 CTPA scans were avoided with a 4.8% false negative rate using age-adjusted D-dimer.
- All patients with D-dimer above the threshold had confirmed pulmonary embolism.

## Abstract

Introduction

Computerized tomography pulmonary angiography (CTPA) is the gold standard test for diagnosing pulmonary embolism (PE); however, owing to its high cost, contrast dye toxicity, and radiation exposure, it is necessary to ensure that CTPA is not performed unnecessarily. European Society of Cardiology (ESC) guidelines recommend the use of pretest clinical assessment tools and D-dimer levels to stratify patients according to risk, thereby determining the need for CTPA. However, such a broad classification incorporates a large population, where cases of CTPA could have been avoided. In this retrospective observational study, we established a new cohort within the moderate risk group, using a D-dimer threshold of 1000 ng/dL, and thereby aimed to identify its impact in identifying the number of cases of CTPA and to identify the number of cases of PE missed.

Materials and methods

This observational retrospective study was conducted over 24 months in the emergency medicine department of a tertiary care hospital in south India. Our study comprised of evaluation of electronic medical records of patients based on the inclusion and exclusion criteria. An overall 182 samples (n=182) were recruited, past medical records were evaluated, and classified patients were classified according to the new stratification criteria based on the Wells score, pulmonary embolism rule-out criteria (PERC), and D-dimer levels. The study outcomes of the number of CTPA cases avoided and PE cases missed out were analyzed.

Results

Assuming all samples (n=182) fell into the low moderate risk (LMR) category (Wells score<2 and PERC positive, or Wells score 2-4), 95 cases (n=95, 95/182) had D-dimer<1000 ng/dL, among which PE was absent in 93 patients (n=93, 97.89%) and present in 2 patients (n=2, 2.10%). The remaining 87 (n=87, 87/182) had D-dimer≥1000 ng/dL where all cases had PE (n=87, 100%). Assuming the samples (n=182) fell into the high moderate risk (HMR) category (Wells score 4.5-6), 21 cases (n=21, 21/182) had D-dimer< age-adjusted D-dimer (AADD) among which, PE was absent in 20 patients (n=20, 95.23 %) and present in 1 patient (n=1, 4.76%). The remaining 161 (n=161, 161/182) had D-dimer≥AADD, whereas all cases had PE (n=161, 100%). The false negative rates were 2.1% in LMR and 4.8% in HMR, while the reduction in CTPA procedures amounted to 95 scans in LMR and 21 scans in HMR.

Conclusions

We identified that establishing a higher threshold of D-dimer (1000 ng/dL) was effective in determining the need for CTPA and potentially reducing the number of CTPAs performed in suspected cases of PE.