Cerebrospinal Fluid Analysis

James W. Myers

Sunanda Mangraj

INTRODUCTION

Cerebrospinal fluid (CSF) analysis is a very important diagnostic tool in evaluation of central nervous system (CNS) infections and other conditions.

CSF is usually obtained from the lumbar subarachnoid space via a spinal tap at the L3-4 or L4-5 interspace, with the patient in the lateral decubitus position. In some cases, it is easier to perform lumbar puncture (LP) with the patient in a sitting position.

Contraindications to Performing a Lumbar Puncture

Increased intracranial pressure
Possible intracranial mass, especially in the posterior fossa
- A spinal tap can precipitate herniation of the tonsils of the cerebellum through the foramen magnum.
- If you suspect a possible intracranial mass lesion, or if you find papilledema during a patient’s exam, LP should be deferred until CT of the head has ruled out possible herniation. Seizures and focal neurologic signs are suggestive of a mass lesion.
Vertebral changes such as scoliosis are a relative contraindication.
Infection at the site of LP overlying skin or epidural abscess can introduce the organism into the underlying subarachnoid space.
Coagulation disorders (<50,000 platelets, patients with hemophilia, vitamin K deficiency)
- Bleeding may complicate the procedure.
- LP should be performed under these circumstances only if necessary.
Improper specimen handling
- Cells will begin to lyse within an hour of collection. Transport as soon as possible to the laboratory.
- Refrigeration helps to slow this process.
- If there is going to be a significant delay, the fluid should be transported in ice.

Opening Pressure

CSF opening pressure (OP) is measured in the lateral decubitus position with the legs and neck in a neutral position. The meniscus will fluctuate between 2 and 5 mm with the patient’s pulse and between 4 and 10 mm with respirations.
Normal OP in adults ranges from 70 to 180 mm of H₂O.
Elevated pressure would clearly be over 200 to 250 mm for most patients.
- Normal OP may be up to 250 in some obese patients.
Straining or coughing can falsely increase OP.
Hyperventilation may lower OP.
OP over 200 mm of H₂O is suggestive of elevated intracranial pressure, which can be seen in many conditions including meningitis, subarachnoid hemorrhages, and space-occupying lesions.
When OP is found to be elevated, slowly remove just enough CSF until pressure reaches 50% of the original OP.

Appearance

Normal CSF is clear.
Greater than 200 white blood cells (WBCs) per mm³, or 400 red blood cell (RBC) per mm³ will give CSF a turbid appearance.
Xanthochromia (yellow, orange, or pink discoloration) is present in the majority of cases of subarachnoid hemorrhage and in a few other conditions as listed in Table 16-1.
- If the RBCs found after a spinal tap are from a bleed, they would have been present long enough to be metabolized into the yellow-green pigment bilirubin, as well as oxyhemoglobin and methemoglobin, unlike those that are only present because of a traumatic tap.
- Discoloration begins after RBCs have been in spinal fluid for at least 2 hours, and remains for 2 to 4 weeks.

Cell Counts

Normal cell count in CSF is up to 5 WBC per mm³ in adults. Please see Table 16-2 for a comparison of cell counts by cause of meningitis.

In neonates, it can be as many as 20 WBC per mm.
As many as 30% of patients may exhibit CSF pleocytosis after a generalized or focal seizure. Generally these counts are <10 to 20 cells.
Over 90% of the patients with bacterial meningitis will have more than 100 mm³ WBC with neutrophil predominance.
WBC counts <100 mm³ are seen more commonly in patients with viral meningitis.

Estimated correction for WBCs from a traumatic tap.

Peripheral blood in CSF after traumatic tap will result in artificial increase in CSF WBC count by 1 WBC for every 750 to 1,000 RBC.

Table 16-1 Cerebrospinal Fluid Appearance and Associated Conditions

Color of CSF Supernatant	Conditions or Causes
Yellow	(Xanthochromia) Blood breakdown products CSF protein >150 mg/dL >100,000 RBCs
	Hyperbilirubinemia
Orange	Blood breakdown products
	Excessive carrot ingestion
Pink	Blood breakdown products
Green	Hyperbilirubinemia, purulent CSF
Brown	Melanomatosis of the meninges.

Table 16-2 CSF Findings in Various Types of Meningitis

Test	Bacterial	Viral	Fungal	Tubercular
Opening Pressure	Usually elevated	Usually normal	Variable	Variable
WBC Count / mm³	500-10,000, usually >1,000	6-1,000,usually <100	Variable	Variable
Cell Differential	PMN predominance^a	Lymphocytic predominance^b	Lymphocytic predominance^b	Lymphocytic predominance^b
Glucose Level	Usually low <40 mg/dL.	Normal Mumps may be low.	Low	Low
Protein Level	Elevated>45mg/dL	Normal to elevated	Elevated	Elevated
^aLymphocytosis present in 10%. ^bPMN may predominate early in the course.

Formula for added WBCs
- WBCs added = WBC (blood) × RBC(CSF) /RBC(blood).
- The blood WBC count is multiplied by the ratio of the cerebrospinal fluid RBC count to blood RBC count.
- The result is the number of artificially introduced WBCs.
- The true CSF white cell count is then calculated by subtracting the artificially introduced WBCs from the actual CSF WBC count.
If the RBC count remains stable in three consecutive tubes, then it is most likely secondary to subarachnoid bleed.
- A falling count is attributed to a traumatic tap.
- A higher RBC count in CSF can also be seen in herpes simplex virus (HSV) encephalitis.