Determination of a Testing Threshold for Lumbar Puncture in the Diagnosis of Subarachnoid Hemorrhage after a Negative Head Computed Tomography: A Decision Analysis

Objective The objective was to determine the testing threshold for lumbar puncture ( LP ) in the evaluation of aneurysmal subarachnoid hemorrhage ( SAH ) after a negative head computed tomography ( CT ). As a secondary aim we sought to identify clinical variables that have the greatest impact on this threshold. Methods A decision analytic model was developed to estimate the testing threshold for patients with normal neurologic findings, being evaluated for SAH , after a negative CT of the head. The testing threshold was calculated as the pretest probability of disease where the two strategies ( LP or no LP ) are balanced in terms of quality‐adjusted life‐years. Two‐way and probabilistic sensitivity analyses ( PSA s) were performed. Results For the base‐case scenario the testing threshold for performing an LP after negative head CT was 4.3%. Results for the two‐way sensitivity analyses demonstrated that the test threshold ranged from 1.9% to 15.6%, dominated by the uncertainty in the probability of death from initial missed SAH . In the PSA the mean testing threshold was 4.3% (95% confidence interval = 1.4% to 9.3%). Other significant variables in the model included probability of aneurysmal versus nonaneurysmal SAH after negative head CT , probability of long‐term morbidity from initial missed SAH , and probability of renal failure from contrast‐induced nephropathy. Conclusions Our decision analysis results suggest a testing threshold for LP after negative CT to be approximately 4.3%, with a range of 1.4% to 9.3% on robust PSA . In light of these data, and considering the low probability of aneurysmal SAH after a negative CT , classical teaching and current guidelines addressing testing for SAH should be revisited.