Normal CSF Values

Abnormal CSF Appearance

Normal CSF is crystal clear. Cloudy CSF is abnormal. It takes at least 200 WBC/mm³ or 400 RBC/mm³ before the CSF becomes cloudy. CSF xanthrochroma is seen when the CSF protein is >150 mg/dl, hyperbilirubinemia, and a bloody tap when enough serum is present to color the CSF (there needs to be at least 100,000 RBC/mm³ for this to occur). After a subarachnoid hemorrhage the CSF will first be a rose color then it will turn yellow after about 10 hours.

Traumatic LP vs. Subarachnoid Hemorrhage

A traumatic or bloody tap is defined as the presence of blood in the CSF that comes from venous blood as the tap is being performed. When blood is in the CSF it is important to distinguish between a bloody tap and a subarachnoid hemorrhage. This can be done  by comparing the 1^st tube vs. the 3^rd tube of CSF. With a bloody tap there is clearing of the CSF. When the CSF is centrifuged the supernatant fluid is also clear. When a subarachnoid hemorrhage has occurred there isn’t any clearing from the 1^st to the 3^rd tube. Within 2-4 hours of the hemorrhage, the red blood cells  will start to lysis which colors the supernatant CSF pink for the first 2-10 hours then the fluid will turn yellow. This xanthrochroma will be at a maximum 48 hours after the hemorrhage and can persist for 2-4 weeks.

Abnormalities of CSF Glucose

An increased level of CSF glucose is of little diagnostic significance other than indicating hyperglycemia. Decreased CSF glucose level (when the CSF glucose is low this is hypoglycorrhachia) is an important finding. The CSF glucose level should be at least 60% of the serum level and the level is usually 45 mg/dl or greater. A serum glucose level should be obtained prior to the LP in order to compare with the CSF value.  The major factors that cause hypoglycorrhachia are increased glucose utilization due to increased anaerobic glycolysis by adjacent brain and spinal cord tissue and decreased function of the membrane glucose transporter.  Low CSF glucose in the absence of hypoglycemia indicates a diffuse generalized meningeal disorder. Hypoglycorrhachia syndromes include the following:

• Acute bacterial meningitis
• Tuberculous and fungal meningitis
• Carcinomatous meningitis
• Viral meningitis- herpes simplex
• Subarachnoid hemorrhage
• Rheumatoid meningitis
• Hypoglycemia

 

 

Abnormalities of CSF Protein

The CSF protein is 1/200th  the concentration of serum protein. The major CSF protein is albumin which comes from the systemic circulation as it is only produced in the liver. The relative concentrations of prealbumin (transthyretin) and transferrin are higher in the CSF then in the serum. Most of the IgG that is in the CSF also comes from the systemic circulation but in multiple sclerosis and certain inflammatory conditions IgG can be produced within the CSF space.

Increased CSF protein is an important but nonspecific indicator of disease. It can be caused by increased permeability of the blood-brain and/or blood-CSF barrier and by decreased CSF absorption or turnover. The degree of CSF elevation can be classified as moderate, great or very great. Diseases in which there is increased CSF protein include vasogenic edema of the brain, hydrocephalus, meningitis, polyneuritis like Guillain-Barre syndrome, subarachnoid hemorrhage and spinal cord block.

Increased White Blood Cells in the CSF
White cells that are present in the CSF are mainly lymphocytes that are derived from the systemic circulation. White cells are cleared from the CSF by lysis and by transport across the arachnoid villi. CSF white cell counts should be done promptly because the white cells will start to lysis within one to two hours at room temperature. Normal CSF should have 5 or less white cells/mm³ and they should be mononuclear cells. If the tap is bloody, the white cell count will be elevated by additional cells from the blood.  A useful rule of thumb to correct this addition is to subtract from the count 1 WBC/mm³ for every 700 RBC/mm³ (assuming a normal hematocrit and leukocyte count).

An abnormal white cell count is greater than 5/mm³ and this is called pleocytosis. Polymorphonuclear leukocytes are rarely found in normal CSF. They can be seen early in viral meningitis but usually the pleocytosis in viral meningitis turns to predominantly mononuclear cells within 12 to 72 hours. They can also be seen with trauma, hemorrhage, infection and meningeal reactions. Marked polymorphonuclear pleocytosis usually signifies bacterial meningitis.