Cannabis Positive Drug Screen Linked to Higher Blood Clot Risk in Older Trauma Patients

A propensity-matched registry study of nearly 3,000 geriatric trauma patients finds that those who tested positive for THC on admission had roughly triple the rates of deep vein thrombosis and pulmonary embolism compared to matched THC-negative controls, though the observational design leaves the question of causality firmly unanswered.

Why This Matters

Geriatric trauma patients already face elevated venous thromboembolism risk due to age, immobility, and injury severity. As marijuana legalization expands across the United States, cannabis use among older adults is rising sharply, yet clinicians have almost no evidence base to guide them on whether THC exposure modifies clotting risk in this population. This study arrives at a moment when trauma teams increasingly encounter THC-positive older patients and need data, however preliminary, to inform screening and prophylaxis decisions.

Clinical Summary

Venous thromboembolism remains a significant cause of morbidity and mortality in geriatric trauma, and emerging preclinical and epidemiological data have raised the possibility that cannabinoids influence coagulation pathways. A 2022 retrospective cohort study by Hosseinzadeh and colleagues, published in the Journal of Surgical Research, used the American College of Surgeons Trauma Quality Improvement Program (TQIP) registry from 2017 to examine whether a positive THC drug screen within 24 hours of admission was associated with thromboembolic complications in patients aged 65 and older. The authors employed 1:2 propensity score matching across more than 30 clinical and demographic variables, including injury severity, comorbidities, chronic anticoagulant use, and the timing of VTE prophylaxis, to construct a matched cohort of 2,835 patients.

In the matched analysis, THC-positive patients had significantly higher rates of deep vein thrombosis (2.2% versus 0.6%, p less than 0.01) and pulmonary embolism (1.5% versus 0.4%, p equals 0.01), with an overall thromboembolic complication rate of 3.0% versus 1.7% (p equals 0.01). Notably, no significant differences emerged for stroke, myocardial infarction, ICU length of stay, hospital length of stay, or mortality. The association with DVT persisted even among patients who received VTE prophylaxis. Key limitations include the retrospective design, the inability of drug screens to distinguish acute from chronic use or quantify dose, the low absolute event counts that widen confidence intervals, and the possibility of unmeasured confounding. The authors appropriately call for prospective studies to determine whether the association is causal and whether targeted prophylaxis strategies are warranted.

Dr. Caplan’s Take

This study asks an important question at exactly the right time. Older adults are the fastest-growing segment of cannabis users, and trauma teams are seeing more THC-positive screens in this age group. The propensity matching is impressively thorough for a registry study, and the signal for venous thromboembolism is consistent across DVT and PE. But a drug screen is a blunt instrument. It tells you that someone was exposed to THC at some point in recent weeks, not that they were intoxicated, not how much they used, and certainly not that THC caused a clot. Patients who ask me whether their marijuana use puts them at risk for blood clots deserve an honest answer: we do not yet know.

In practice, I do not change VTE prophylaxis protocols based on THC screen status alone. What I do is ensure that every geriatric trauma patient receives guideline-concordant thromboprophylaxis regardless of cannabis history, and I use a positive screen as an opportunity to document substance use more carefully and to discuss it without judgment. If prospective data eventually confirm this association, we will need to revisit prophylaxis thresholds, but we are not there yet.

Clinical Perspective

This study sits early in the research arc for cannabis and thromboembolism. It builds on scattered case reports and small series that have suggested a prothrombotic effect of cannabinoids, but it is the first to examine the question specifically in a large geriatric trauma population using rigorous matching methodology. It confirms that the association warrants serious investigation while simultaneously demonstrating why observational registry data alone cannot resolve the question. Clinicians should not interpret these findings as evidence that THC causes clots, nor should they dismiss the signal. The finding that chronic anticoagulation appeared to attenuate the association is intriguing but based on a small subgroup and should be considered hypothesis-generating only.

From a pharmacological standpoint, THC interacts with the endocannabinoid system in ways that may influence platelet aggregation and endothelial function, but the specific prothrombotic mechanism, if one exists, remains uncharacterized. Drug interactions between cannabis and anticoagulants such as warfarin are well documented and could complicate prophylaxis dosing in THC-positive patients. For now, the most actionable recommendation is to ensure that a positive THC screen triggers a thorough medication reconciliation and a documented substance use history, while maintaining standard VTE prophylaxis protocols without modification pending stronger evidence.

Study at a Glance

Study Type

Retrospective cohort with 1:2 propensity score matching

Population

Geriatric trauma patients (age 65 and older) at ACS Level I/II trauma centers

Intervention

Positive THC drug screen within 24 hours of admission (exposure of interest)

Comparator

Propensity-matched THC-negative geriatric trauma patients

Primary Outcomes

Thromboembolic complications: DVT, PE, stroke, myocardial infarction

Sample Size

2,835 matched patients (945 THC-positive, 1,890 THC-negative) from 286,242 eligible

Journal

Journal of Surgical Research

Year

2022

DOI or PMID

Published in Journal of Surgical Research, 2022

Funding Source

Not reported

What Kind of Evidence Is This

This is a retrospective cohort study drawn from a large national trauma registry, strengthened by propensity score matching across more than 30 covariates. It occupies a mid-tier position in the evidence hierarchy, above case series and unmatched observational studies but below prospective cohort studies and randomized trials. The most important inference constraint is that propensity matching can only adjust for measured variables, meaning that unmeasured confounders such as patterns of substance use, lifestyle factors, or reasons for selective drug screening may account for part or all of the observed association.

How This Fits With the Broader Literature

Prior evidence linking cannabis to thromboembolic events has been limited primarily to case reports and small case series, many involving younger patients with arterial events such as stroke or myocardial infarction rather than venous thromboembolism. This study extends the literature by providing the first large-scale, propensity-matched analysis in an older trauma population and by focusing specifically on DVT and PE. The finding that venous rather than arterial events drove the association is noteworthy and somewhat unexpected given the existing case literature.

The results are broadly consistent with preclinical work suggesting that cannabinoids may modulate platelet function and endothelial activation, though the translational relevance of those findings remains uncertain. A key gap is the absence of any prospective study or randomized trial examining THC exposure and VTE risk in any population, making this study hypothesis-generating rather than confirmatory.

Common Misreadings

The most likely overinterpretation is reading this study as evidence that marijuana use causes blood clots in older adults. The study’s own title and conclusion lean toward causal language, stating that THC “increases the risk” of thromboembolic complications, but the retrospective observational design does not support that claim. A positive drug screen reflects prior exposure over days to weeks and cannot distinguish a daily heavy user from someone who used cannabis once two weeks before injury. Additionally, only 0.33% of the eligible population tested THC-positive, which strongly suggests that drug screening was not routine and that the THC-positive group may differ from the general geriatric trauma population in unmeasured ways that independently affect clotting risk.

Bottom Line

This study identifies a statistically significant association between a positive THC drug screen and higher DVT and PE rates in propensity-matched geriatric trauma patients. It does not establish that cannabis causes thromboembolism. The findings are hypothesis-generating and warrant prospective investigation, but they do not justify changes to current VTE prophylaxis protocols based on THC screening status alone. Clinicians should use positive screens as a prompt for thorough substance use documentation and medication reconciliation.

References

1. Hosseinzadeh P, et al. Cannabis positive drug screen linked to higher blood clot risk in older trauma patients. Journal of Surgical Research. 2022. ACS TQIP 2017 database analysis.