August 1986

Pitfalls in Interpretation of Traumatic Lumbar Puncture Formula

Author Affiliations

Children's Service Massachusetts General Hospital 32 Fruit St Boston, MA 02114

Am J Dis Child. 1986;140(8):737-738. doi:10.1001/archpedi.1986.02140220019006

Sir.—A traumatic lumbar puncture obscures the cerebrospinal fluid's original white blood cell (WBC) count. According to recent publications,1-3 the calculation for estimating the contribution of contaminating peripheral blood to the total WBC count of a traumatic spinal tap can be represented as follows: WBCcontaminating = RBCCSF × WBCperipheral/RBCperipheral, in which RBC indicates red blood cells and CSF indicates cerebrospinal fluid. The following discussion demonstrates that the proposed calculation does not permit the clinician to rule out meningitis, which is defined as six or more leukocytes per cubic millimeter (≥×106/L) in the CSF.1

If the patient's peripheral WBC/RBC ratio is estimated (published estimates range from 1/300 to 1/7002-4), the calculated contaminating WBC count may vary by more than 100% depending on the ratio used. Table 1 illustrates the different clinical conclusions that could result. Therefore, estimated ratios should not be

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