Interpreting red blood cells in lumbar puncture: distinguishing true subarachnoid hemorrhage from traumatic tap

The study purpose was to determine the optimal use of lumbar puncture (LP) red blood cell (RBC) counts to identify subarachnoid hemorrhage (SAH) when some blood remains in the final tube.

A case series was performed at a tertiary emergency department (ED). Records of 4,496 consecutive adult patients billed for LPs between 2001 and 2009 were reviewed. Inclusion criteria were headache (HA), final tube RBCs ≥ 5, and neurovascular imaging within 2 weeks of the visit. Demographics, relevant history and physical examination components, LP results, and neuroimaging findings were recorded for 280 patients. True-positive (TP) and true-negative (TN) SAH were strictly defined. Primary outcomes were the areas under the receiver operating characteristic curves (AUC) for final tube RBC count, differential RBC count between the final and initial tubes, and absolute differential RBC count between the final and initial tubes divided by the mean RBC count of the two tubes (also called the percent change in RBC count).

There were 26 TP and 196 TN results; 58 patients were neither. The TP group consisted of 19 patients with visible or possible SAH on imaging (17 on noncontrast head computed tomography [CT; 12 definite and five possible] and two on magnetic resonance imaging), six with xanthochromia and a vascular lesion (aneurysm or arteriovenous malformation [AVM] > 2 mm), and one with xanthochromia and polymerase chain reaction (PCR)-positive meningitis. As a test for SAH, final tube RBC AUC was 0.85 (95% confidence interval [CI] = 0.80 to 0.91). Interval likelihood ratios (LRs) for final tube RBC count were LR 0 (95% CI = 0 to 0.3) for RBCs < 100, LR 1.6 (95% CI = 1.1 to 2.3) for 100 < RBCs < 10,000, and LR 6.3 (95% CI = 4.8 to 23.4) for RBCs > 10,000. Differential RBC count was not associated with SAH, with AUC 0.45 (95% CI = 0.31 to 0.60). However, the percent change in RBC count between the final and initial tubes had an AUC 0.84 (95% CI = 0.78 to 0.90), and the optimal test threshold for SAH was 0.63, with positive LR 3.6 (95% CI = 2.7 to 4.7) and negative LR 0.10 (95% CI = 0.03 to 0.4) for percent change <63% and >63%, respectively. This test added additional independent information to the final tube RBC count based on improved logistic regression model fit and discriminatory ability as measured by the LR test and c statistic, respectively.

Final LP tube RBC count and the percent change in RBC count, but not the simple differential count between the final and initial tubes, were associated with SAH. In this sample, there were no patients with SAH who had RBCs < 100 in the final tube, and RBCs > 10,000 increased the odds of SAH by a factor of 6.