Nothing sparks controversy quite like a discussion on the utility of thrombolytics. No sooner had the wave of debate brought on by the publication of the PEITHO trial and its finding of no overall mortality benefit died down, did JAMA stoke these flames with the publication of a meta-analysis including the entirety of the literature on thrombolytic use for pulmonary embolism. Examining 16 trials, the authors found a statistically significant absolute mortality benefit of 1.12% or an NNT of 59 patients. This benefit was offset by the increase in major bleeding events observed in those given thrombolytics (9.24% vs 3.42%) with a 1.27% absolute increase in ICH.
The cascade of incendiary events continued when one week later a second meta-analysis examining the very same question was published in Journal of Thrombosis and Haemostasis. Only these authors claimed to find the exact opposite of their JAMA counterparts. In this case the authors found no statistical improvement in mortality in the patients given thrombolytics when compared to those given a placebo. Despite these contradictory claims, a more comprehensive inspection reveals these meta-analyses are extensively saying the same thing. A comparison of the two serves as a timely reminder that conclusions reached from any meta-analysis is primarily dependent on the trials selected for inclusion. The JAMA meta-analysis included 2115 patients in 16 trials, while the Journal of Thrombosis and Haemostasis examined only 1510 patients in 6 trials. Interestingly, the absolute risk reduction in mortality was 1.12% in the JAMA publication vs 1.4% in the Journal of Thrombosis and Haemostasis publication. Though the JAMA analysis had an overall smaller absolute risk reduction, the result reached statistical significance due to the larger sample size.
More importantly the results of these publications should not come as a surprise. Before the publication of PEITHO the data on thrombolytics for PE was sparse. Most of the trials suffered from small sample sizes and questionable methodology. It is the amalgamation of these small trials that accounts for the mortality benefit in both meta-analyses. In the JAMA publication the mortality difference consisted of 17 fewer deaths in the thrombolytic arm compared to the placebo. All of which originated from these small underpowered studies. Conversely, the two large high quality trials (PEITHO and MSPPE) consisting of 1005 and 256 patients respectively made up the majority of patients meta-analyzed, neither of which found a mortality benefit with the use of thrombolytics. Moreover the 2% absolute increase in ICH seen in the PEITHO cohort is only diluted by the inclusion of these small trials, whose sample sizes were not powered to detect such rare events. A more elegant design would be to utilize a weighted average or one of the various statistical methods that takes into account each study’s sample size and event rate, allocating a greater weight to the hardier cohorts. Though one might argue an equally elegant solution would be to not include such flawed trials in the first place.
The publication of these dueling meta-analyses highlights the flaws of such statistical endeavors. Small trials with flawed methodological designs are prone to fall victim to publication bias and the fluctuating whims of chance. Collecting this data and attaching a statistical judgment to it does not correct these imperfections, it augments them. The overall benefit of thrombolytics in PE is yet undetermined, but the answer will not be elucidated in such analysis. We require further large randomized controlled trials like the PEITHO trial. Adding small flawed cohorts to this robust dataset does nothing but muddy the already murky waters.
“Thrombolysis for pulmonary embolism and risk of all-cause mortality, major bleeding, and intracranial hemorrhage: a meta-analysis.”
“Impact of the efficacy of thrombolytic therapy on the mortality of patients with acute submassive pulmonary embolism: a meta-analysis.”