What I Have Learned About Infectious Diseases With My Sleeves Rolled Up

Karen L. Roos, MD

Disclosures

Semin Neurol. 2002;22(1) 

In This Article

The Secrets of Cerebrospinal Fluid Interpretation

In children and adults, the normal cerebrospinal fluid (CSF) white blood cell (WBC) count is in the range of 0 to 5 mononuclear cells (lymphocytes and monocytes) per mm3. The presence of more than 5 WBCs/mm3 is abnormal. In normal uninfected CSF, there should be no polymorphonuclear leukocytes; however, with the use of the cytocentrifuge, an occasional polymorphonuclear leukocyte may be seen. If the total WBC count is less than 5, the presence of a single polymorphonuclear leukocyte may be considered normal.[1,2,3]

Dr. James Corbett, Professor and Chairman of Neurology at the University of Mississippi, taught me early in my career that "he who _______ up the lumbar puncture, _______ up the case." That is especially true in the case of a traumatic tap. Blood is introduced into the CSF during lumbar puncture when the spinal needle penetrates the radicular arterial vessels and veins. To determine the true number of white blood cells in the CSF, 1 WBC/mm3 for every 700 red blood cells (RBCs)/mm3 is subtracted from the total WBC count.[4] When the lumbar puncture has been traumatic, the CSF protein concentration will be increased by 1 mg/dL for every 1000 RBCs/mm3.[2] The Venereal Disease Research Laboratory (VDRL) test and the polymerase chain reaction (PCR) test for herpes simplex virus (HSV) and Epstein Barr virus (EBV) should be interpreted cautiously when the lumbar puncture has been traumatic. When the CSF is contaminated with blood, a false-positive VDRL may occur. A false-negative HSV CSF PCR may occur when the CSF specimen contains erythrocytes or their hemolyzed products. As EBV DNA is found in peripheral blood mononuclear cells, contamination of spinal fluid by blood could result in a false-positive CSF PCR for EBV DNA.

The upper range of normal for the lumbar CSF protein concentration in the adult is 50 mg/dL. An increased CSF protein concentration is a nonspecific abnormality, seen in any process that disrupts the blood-brain barrier permeability. The CSF protein concentration is increased in any number of diseases of the nervous system, including infections, demyelinating disorders, stroke, diabetes complicated by cerebrovascular disease, autoimmune disorders, and with aging. An increased CSF protein concentration has the most diagnostic significance when it is markedly increased, in the range of 2000 to 3000 mg/dL. In this situation, be suspicious of carcinomatous meningitis.

A CSF opening pressure should be measured in all patients with headache and in every patient with an altered level of consciousness. The upper limit of normal CSF opening pressure with the patient in the lateral recumbent position is 110 mm H2O in infants, 150 mm H2O in children, 180 mm H2O in adults, and 250 mm H2O in obese adults.[5]Table 1 provides a list of the general diagnostic studies to send on CSF. For central nervous system (CNS) fungal infections, there are CSF antigen studies. For CNS viral infections there are CSF PCR assays and antibody titers. The interpretation of CSF viral antibody titers require paired sera. Cerebrospinal fluid viral antibody titers are particularly helpful in the diagnosis of HSV encephalitis and varicella zoster virus (VZV) encephalitis. In patients with HSV encephalitis, the intrathecal synthesis of HSV-specific antibodies can be detected usually within 8 to 12 days or more after the onset of symptoms, and remain positive for at least as long as 30 days and, in some series, as long as 3 months after the onset of neurological symptoms. The HSV antibody assay is performed on serum and CSF; a less than 20:1 serum-to-CSF ratio of antibodies to HSV-1 suggests intrathecal synthesis of antibodies.[6] The demonstration of VZV immunoglobulin M or immunoglobulin G in CSF is useful in making the diagnosis of varicella zoster encephalitis.

In patients with encephalitis, it is important to distinguish between HSV-1 DNA in CSF and HSV-2 DNA in CSF. Herpes simplex virus-2 can reactivate in the setting of fever and may be detected by PCR in the CSF of patients with other etiologies for their encephalitis.

Cerebrospinal fluid that is to be sent to the PCR laboratory does not really require special handling, although many laboratories today want the CSF specimen to be sent on ice. This request has resulted in my finding tubes of CSF sitting in Styrofoam cups of warm water throughout the hospital. My residents tell me that these are the remains of the battle they fought the night before. Why, then, do laboratories request that CSF be sent on ice if this is unnecessary? I suspect this comes from the study by Lakeman and Whitley and the National Institute of Allergy and Infectious Diseases Collaborative Antiviral Study Group[7] that detected HSV DNA by PCR in CSF of 53 (98%) of 54 patients with biopsy-proven HSV encephalitis. The CSF specimens had been obtained from patients enrolled in studies of HSV encephalitis between 1974 and 1979 that investigated vidarabine therapy, and in a clinical trial done between 1981 and 1987. CSF from all patients were stored at -20°C; however, specimens had been thawed and refrozen several times to evaluate other noninvasive methods for the diagnosis of HSV encephalitis.[7] If DNA can be analyzed in bodies that are exhumed in criminal investigations, it is not likely that DNA is going to vanish from CSF that is not kept on ice during transportation to the laboratory. It is a fact, however, that WBCs disintegrate in CSF that sits around for more than 4 hours before being analyzed in the laboratory. This is one of the most common reasons for normalization of the CSF WBC count in a patient who has a progressive clinical course.

The latex particle agglutination test for bacterial antigens of Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae type b, group B streptococcus, and Escherichia coli K1 strains in the CSF is very useful for making a rapid diagnosis of bacterial meningitis, especially in those patients who have been pretreated with antibiotics and in whom CSF Gram's stain and culture are negative. There is not yet a rapidly available CSF PCR test for bacterial DNA. These tests are commercially available but it typically takes 4 days or more to get a result; by then the organism has been identified on Gram's stain and grown in culture. Bacterial antigens can be identified by the latex particle agglutination test within an hour of a sample of CSF being received in the laboratory. The latex particle agglutination test is highly specific but not highly sensitive. A positive test is virtually diagnostic of bacterial meningitis by that organism; however, a negative test does not exclude infection by one of these organisms. The cryptococcal polysaccharide antigen is a highly sensitive and specific test for cryptococcal meningitis. Of patients with meningitis due to Histoplasma capsulatum, 40 to 50% will have a positive CSF histoplasma polysaccharide antigen test. The CSF Coccidioides immitis complement fixation antibody test is highly specific for coccidioidal meningitis, as these antibodies do not cross the blood-brain barrier from serum.

The 14-3-3 immunoassay on CSF has been demonstrated to be useful for the diagnosis of Creutzfeldt-Jakob disease in select patients with dementia and myoclonus or ataxia. The list of diseases that cause false-positive results is quite long and includes herpes simplex virus encephalitis, bacterial and tuberculous meningoencephalitis, recent cerebral infarctions, degenerative dementias including Alzheimer's disease, carcinomatous meningitis, paraneoplastic neurologic disorders, anoxic encephalopathy, multiple sclerosis, mitochondrial encephalomyelopathy, cerebral amyloid angiopathy, and Hashimoto's encephalopathy, among others. Present recommendations are that this test not be used for screening purposes and should only be used in patients that meet the diagnostic criteria for Creutzfeldt-Jakob disease (classic clinical presentation and course and electroencephalographic abnormalities).[8]

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