Again With the Failings of CTPA

Most of the unhinged ramblings on this blog involve lamenting the excessive sensitivity of CT pulmonary angiograms for the diagnosis of pulmonary embolism. “Excessively sensitive for PEs of uncertain clinical signifiance!” and “Too many false positives in an inappropriately selected population!” gloomily drones the author (We can’t all, and some of us don’t).

Now, again, come the baffling attacks from the right – the CTPA isn’t sensitive enough:

“The negative predictive value of CTPA for VTE varies according to pretest prevalence of PE, and is likely to be insufficient to safely rule out VTE as a stand-alone diagnostic test amongst patients at the highest pretest probability of VTE. Prospective studies are required to validate the appropriate diagnostic algorithm for this subgroup of patients.”

Foundational quibbles in the narrative induced by their meta-analysis:

  • Sure, maybe, in the cohort of studies before 2006 – but since then, the number of VTE “missed” by CTPA is less than 1%.
  • A VTE “missed” by CTPA includes lower extremity DVT concurrently diagnosed by duplex ultrasound.  Whether a CTPA should be impugned for failing to include the legs is a separate debate regarding the adequacy of its Natural State of Being.
  • Again, a VTE “missed” by CTPA includes all VTE (including LE DVT) diagnosed in the three-month follow-up period, a timeframe certainly adequate for individuals at high-risk for VTE to develop thrombosis anew.
  • “High” clinical probability in this study refers to those patients with ≥40% pretest likelihood of disease, which is tremendously infrequently encountered in clinical practice.

Clearly, these authors are far from convincing me CTPA is guilty of relevant concerns for inadequate sensitivity in these modern times.  One problem at a time, please; queue up, now.

“Outcomes following a negative computed tomography pulmonary angiography according to pulmonary embolism prevalence: a meta-analysis of the management outcome studies”
https://www.ncbi.nlm.nih.gov/pubmed/29645405

One thought on “Again With the Failings of CTPA”

  1. Excellent Ryan.

    Most patients that are determined to be high probability are more likely to have larger PE’s. I.e more risk factors, more symptomatic, tachycardia, dyspnoeic etc.

    There is a problem with this Bayesian approach to the diagnosis of PE with CTPA… spectrum bias.

    If a patient is highly symptomatic from a large PE the CTPA should have 100% sensitivity. Said another way, the test should be positive all the time if a patient has a large PE and/or clot burden.

    If a patient gets a negative CTPA in a highly symptomatic individual, one should really think about other causes…

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