The utility of serum serologic tests for viruses is limited by the time required to obtain results, but they are useful in determining retrospectively the source of meningitis or encephalitis. More rapid tests that use the polymerase chain reaction (PCR) in CSF, which amplifies viral DNA fragments, are now widely available for diagnosis, particularly for herpesviruses, cytomegalovirus, and JC virus. These tests are most useful in the first week of infection, when the virus is being reproduced and its genomic material is most prevalent; after this time, serologic techniques for viral infection are more sensitive. Amplification of DNA by PCR is particularly useful in the rapid detection of tubercle bacilli in the CSF, the conventional culture of which takes several weeks at best. Tests for the detection of 14-3-3 protein that reflects the presence of prion agents in the spinal fluid are available and may aid in the diagnosis of the spongiform encephalopathies, but the results have been erratic (Chap. 32). Testing for anti-Hu and anti-NMDA and other antibodies has become practical for paraneoplastic and non-paraneoplastic encephalitides (Chap. 30).
The average osmolality of the CSF (295 mOsm/L) is identical to that of plasma. As the osmolality of the plasma is increased by the intravenous injection of hypertonic solutions such as mannitol or urea, there is a delay of up to several hours in the rise of osmolality of the CSF. It is during this period that the hyperosmolality of the blood maximally dehydrates the brain and decreases the volume of CSF. Table 2-2 lists the CSF and serum levels of sodium, potassium, calcium, and magnesium. Neurologic disease does not alter the CSF concentrations of these constituents in any characteristic way. The low CSF concentration of chloride that occurs in bacterial meningitis is not specific but a reflection of hypochloremia and, to a slight degree, of a greatly elevated CSF protein. Acid–base balance in the CSF is of interest in relation to metabolic acidosis and alkalosis but pH is not routinely measured. Normally, the pH of the CSF is approximately 7.33—that is, somewhat lower than that of arterial blood, which is 7.41. The PCO2 in the CSF is in the range of 45 to 49 mm Hg—that is, higher than in arterial blood (about 40 mm Hg). The bicarbonate levels of the two fluids are about the same, 23 mEq/L. The pH of the CSF is precisely regulated, and it tends to remain relatively unchanged even in the face of severe systemic acidosis and alkalosis. Acid–base changes in the lumbar CSF do not necessarily reflect the presence of similar changes in the brain, nor are the CSF data as accurate an index of the systemic changes as direct measurements of arterial blood gases.