Once the subarachnoid space has been entered, the pressure and fluctuations with respiration of the CSF are observed, and samples of fluid are obtained. The gross appearance of the fluid is noted, after which the CSF, in separate tubes, can be examined for a number of features. The standard determinations are of the number and type of cells, protein and glucose content, and microscopy and bacterial culture. In addition, the following can be studied: (1) tumor cells (cytology and flow cytometry); (2) presence of oligoclonal bands or content of gamma globulin; (3) serologic (immunological) tests; (4) substances elaborated by some tumors (e.g., β2 microglobulin); and (5) markers pertaining to certain infections such as fungi, cryptococcal and other antigen and India ink preparations, mycobacteria, DNA of herpesvirus, cytomegalovirus and other organisms (by polymerase chain reaction), markers of certain infections (e.g., 14-3-3 protein), and viral isolation.
With the patient in the lateral decubitus position, the CSF pressure is measured by a manometer attached to the needle in the subarachnoid space. In the normal adult, the opening pressure varies from 100 to 180 mm H2O, or 8 to 14 mm Hg. In children, the pressure is in the range of 30 to 60 mm H2O. A pressure above 200 mm H2O with the patient relaxed and legs straightened generally reflects increased intracranial pressure. In an adult, a pressure of 50 mm H2O or below indicates intracranial hypotension, generally caused by leakage of spinal fluid or systemic dehydration (see Avery and colleagues). When measured with the needle in the lumbar sac and the patient in a sitting position, the fluid in the manometer rises to the level of the cisterna magna (pressure is approximately double that obtained in the recumbent position). It fails to reach the level of the ventricles because the latter are in a closed system under slight negative pressure, whereas the fluid in the manometer is influenced by atmospheric pressure. Normally, with the needle properly placed in the subarachnoid space, the fluid in the manometer oscillates through a few millimeters in response to the pulse and respiration and rises promptly with coughing, straining, and with jugular vein or abdominal compression. An apparent low pressure can also be the result of a needle aperture that is not fully within the subarachnoid space; this is evidenced by the lack of expected fluctuations in pressure with these maneuvers.
The presence of a spinal subarachnoid block was in the past confirmed by jugular venous compression (Queckenstedt test, which tests for a rapid rise in CSF pressure after application of the pressure on the vein). The maneuver risks worsening of a spinal block or of raised intracranial pressure and is of historical interest.
Normally, the CSF is clear and colorless. Minor degrees of color change are best detected by comparing test tubes of CSF and water against a white background (by daylight rather than by fluorescent illumination) or by looking down into the tubes from above. The presence of red blood cells imparts a hazy or ground-glass appearance; at least 200 red blood cells (RBCs) per cubic millimeter (mm3) must be present to detect this change. The presence of 1,000 to 6,000 RBCs per cubic millimeter imparts a hazy pink to red color, depending on the amount of blood; centrifugation of the fluid or allowing it to stand causes sedimentation of the RBCs. Several hundred or more white blood cells (WBCs) in the fluid (pleocytosis) may cause a slight opaque haziness.