Clearing of red blood cells in lumbar puncture does not rule out ruptured aneurysm in patients with suspected subarachnoid hemorrhage but negative head CT findings.

Background And Purpose: In evaluating the results of lumbar puncture (LP), a decrease in the number of red blood cells (RBCs)/mm3 between the first and fourth tubes collected (clearing) has often been assumed to indicate a traumatic puncture rather than the presence of subarachnoid hemorrhage (SAH). We tested the hypothesis that, in the setting of severe headache, CSF clearing coupled with an unremarkable unenhanced CT scan was negatively predictive of the presence of aneurysm and could be used to reduce the need for conventional arteriography.

Methods: Cerebral angiography was performed to evaluate suspected SAH in 123 consecutive patients over 2 years at a university teaching hospital. Records of these patients were reviewed. Among the subset without SAH on CT scan, LP results were evaluated for clearing. Clearing was arbitrarily defined as a 25% reduction in RBCs between the first and fourth tubes. This subset's records were also reviewed for the presence of aneurysm at cerebral angiography or at follow-up 6 weeks later. Data were analyzed for correlation between clearing and aneurysm.

Results: Of the 123 patients whose records were reviewed, 22 did not show an SAH on CT scan. Of those 22 patients, eight had aneurysm at angiography and 14 did not. Clinical diagnoses in the other 14 included trauma, herpes meningitis, sickle cell disease, and cocaine use. CSF clearing was noted in 25% of those with an aneurysm (two of 8) and 21% of those without an aneurysm (three of 14). In the two cases with aneurysms, RBCs cleared from 3550 to 2550 (-28%) and from 24,686 to 17,842 (-28%), respectively. In the remaining six cases with aneurysms, RBCs increased a mean of 1370% between the first and fourth tubes (range, -22% to 7700%). Two of these six had a reduction that did not meet our criteria for clearing (-22% and -5.3%, respectively). In the 14 cases without aneurysms, RBCs increased a mean of 70% between the first and fourth tubes (range, -99% to 895%). In addition to the three of these 14 that met our criteria for clearing (-99%, -99%, and -43%), four cases had a reduction that did not (range, -0.7% to -14%).

Conclusion: A 25% reduction in RBC concentration between the first and fourth tubes of CSF in patients with suspected SAH but negative CT findings occurs even in cases of ruptured aneurysms. Formal evaluation for the presence of an aneurysm is still necessary in this scenario.