You've now breathed a sigh of relief, and managed to obtain four vials of CSF. Now what? Well, you need to send them to the lab for analysis. The recommended studies include (this order can be changed):

• Tube#1 : Gram stain, Culture and Sensitivity
• Tube#2 : Glucose, Protein
• Tube#3 : Cell Count and Differential
• Tube#4 : Any special studies you require (fungal/viral/chemical studies)

In order to interpret your results, it is important to know about the normal CSF content.

The following table outlines the values seen in a normal LP:

Recall that the opening pressure is measured at the start of your LP with the pressure manometer. This pressure is only valid in the lateral decubitus position. Increased opening pressures suggest increased intracranial pressures from a mass lesion (neoplasm, hemorrhage or cerebral edema), overproduction of CSF (choroid plexus papilloma) or a defective outflow mechanism through the ventricles.

Normal CSF is crystal clear in appearance, yet up to 400 cells/mm³ can reside in the CSF and the physician will not see changes in the clarity of the CSF. There are two major reasons for cloudy CSF. The first is the presence of large numbers of WBC's. In CSF infection, the CSF can appear turbid as the number of WBC's increases. They accumulate to the point of making the CSF appear cloudy or it can even appear as pus. The second reason for CSF discoloration is due to red cells and their breakdown products. Large numbers of RBC's in the CSF can make the CSF appear very bloody. After the blood has been in the CSF for greater than 12 hours, the red cells begin to lyse in large quantities and the oxyhemoglobin and bilirubin cause a yellow orange discoloration of the CSF. This orange red discoloration is known as xanthochromia and can be measured in the lab by spectrographic analysis. Formation of the RBC breakdown products peaks about 24 hours after blood enters the CSF and resolves in 3 – 30 days. The presence of xanthochromia is always pathological.

Normal CSF is allowed to have up to 5 RBC's per mm³, albeit it is common to find no RBC's in the CSF. Levels higher than this suggests either SAH, intracranial bleed or traumatic tap. A traumatic tap occurs when the LP needle enters a blood vessel while performing the procedure. Traumatic taps commonly occur when the needle has advanced slightly too far and transfixed the internal vertebral plexus (the more densely packed area of vasculature on the ventral side of the spinal cord). Differentiating between traumatic tap and SAH is usually fairly easy. If you suspect a traumatic tap, order cell counts on test tubes one and three. As the CSF washes the needle, the blood will also be washed out and the number of RBC's should decrease. Also have the lab examine the CSF for xanthochromia. Because the traumatic tap is acute, there should be no xanthochromia. The presence of xanthochromia suggests there has been previous CSF bleeding.

The normal CSF contains up to 5 WBC's per mm³. These may be either lymphocytes or monocytes. If the CSF contains more than 5 WBC's or other cell lines, infection is likely. The most worrisome of these is acute bacterial meningitis. Bacterial meningitis displays a marked pleocytosis ranging between 500 and 20000 WBC/mm³. The differential of these cells demonstrates mostly neutrophils. Meningitis may also be caused by a variety of viruses. The CSF in these cases demonstrates 10 to 1000 WBC's per mm³ with a differential of mostly lymphocytes and monocytes.

CSF glucose is normally 60-70% of the serum values. Glucose enters the CSF by the choroid plexus or through active transport in the capillary membranes. Low levels of glucose are commonly seen in CNS infection and are due to inhibition of the glucose active transport as well as increased utilization of glucose by the brain and spinal cord. Elevated glucose levels are usually inconsequential and suggest serum hyperglycemia.

Causes of CSF Hypoglycemia:

• bacterial meningitis
• tuberculous meningitis
• fungal meningitis
• mumps meningitis
• amebic meningitis
• chemical meningitis
• trichinosis
• syphilis
• herpes encephalitis
• SAH
• meningeal carcinomatosis
• sarcoidosis
• cysticercosis
• hypoglycemia

CSF protein usually runs in the 0.2 – 0.45 g/L range. Most of the proteins are carried in from the blood and delivered to the CSF based on membrane permeability and protein size. Increased protein levels are seen in numerous disease states and especially in meningitis and SAH.

Gram stain is an invaluable tool in suspected bacterial meningitis. Gram negative diplococci are suggestive of N. meningitidis. Small gram negative bacilli may indicate H. influenza. Gram positive cocci are suggestive of S. pneumonia, Streptococcus or Staphylococcus species. Unfortunately, up to 20% of gram stains may be falsely negative as there are not enough organisms to see.

A review of the CSF values in common disorders shows: