Cerebrospinal fluid evaluation

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Chapter 8 CEREBROSPINAL FLUID EVALUATION

Theodore X. O’Connell

General Discussion

Lumbar puncture (LP) is a commonly performed procedure in pediatrics, used most commonly to evaluate for the presence of meningitis. Commonly performed tests on cerebrospinal fluid (CSF) include protein and glucose levels, cell counts and differential, microscopic examination, and culture. Additional tests—such as opening pressure, supernatant color, latex agglutination, and polymerase chain reaction—also may be performed.

Protein concentrations usually are elevated in patients with bacterial meningitis. Values less than 40 mg/dL are considered normal in infants and children. The CSF protein may be elevated in many processes, including infectious, immunologic, vascular, and degenerative diseases as well as tumors of the brain and spinal cord. The CSF protein may be increased after a bloody tap by approximately 1 mg/dL for every 1000 mm³. Values greater than 100 mg/dL suggest that bacterial infection is present. However, protein concentrations of more than 100 to 120 mg/dL commonly are observed in healthy, uninfected newborn infants, especially premature infants.

In most patients with bacterial meningitis, the CSF glucose concentration is low as a result of increased metabolic demands. A CSF glucose concentration that is less than half the simultaneously obtained blood glucose concentration usually is considered abnormal.

Normal white blood cell (WBC) count values depend on the patient’s age. In patients with acute bacterial meningitis, the cell count can be extremely variable, but it is usually in the range of 1000 to 5000 leukocytes/mm³. However, very early in the illness, the cell count may be normal despite a positive CSF culture. Polymorphonuclear (PMN) cells are always abnormal in a child, but 1 to 2/mm³ may be present in a normal neonate. An elevated PMN count suggests bacterial meningitis or the early phase of an aseptic meningitis. CSF lymphocytes indicates aseptic, tuberculous, or fungal meningitis; demyelinating diseases; brain or spinal cord tumor; immunologic disorders (including collagen vascular diseases); and chemical irritation (post myelogram, intrathecal methotrexate).

Normal CSF contains no red blood cells (RBCs). The presence of RBCs indicates a traumatic tap or a subarachnoid hemorrhage. Progressive clearing of blood CSF is noted during collection of the fluid in the case of a traumatic lumbar puncture.

The probability of seeing bacteria on a Gram-stained CSF preparation is dependent on the number of organisms present. The sensitivity is approximately 80% in a properly prepared smear but is lower when Listeria monocytogenes is the cause of meningitis.

Culture should be performed routinely on all spinal fluid specimens, even those that are grossly normal or have normal leukocyte count. The yield of CSF culture is lower in patients previously treated with antibiotics.

The mean opening lumbar CSF pressure is variable, depending on the age of the child. These pressures are outlined in Table 8-1. Opening pressures may exceed 150 to 200 mm H₂0 when bacterial meningitis is present.

Table 8-1 Evaluation of Cerebrospinal Fluid

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	WBC Count	Mean % PMNs
Preterm	0-25 WBCs/mm³	57%
Term	0-22 WBCs/mm³	61%
Child	0-7 WBCs/mm³	5%
Glucose
Preterm	24-63 mg/dL	1.3-3.5 mmol/L
Term	34-119 mg/dL	1.9-6.6 mmol/L
Child	40-80 mg/dL	2.2-4.4 mmol/L
CSF Glucose/Blood Glucose
Preterm	55%-105%

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