Cerebrospinal Fluid Analysis

Lumbar puncture is frequently performed in
primary care. Properly interpreted tests can make cerebrospinal fluid (CSF) a
key tool in the diagnosis of a variety of diseases. Proper evaluation of CSF
depends on knowing which tests to order, normal ranges for the patient's age,
and the test's limitations. Protein level, opening pressure, and CSF-to-serum
glucose ratio vary with age. Xanthochromia is most often caused by the presence
of blood, but several other conditions should be considered. The presence of
blood can be a reliable predictor of subarachnoid hemorrhage but takes several
hours to develop. The three-tube method, commonly used to rule out a central
nervous system hemorrhage after a "traumatic tap," is not completely
reliable. Red blood cells in CSF caused by a traumatic tap or a subarachnoid
hemorrhage artificially increase the white blood cell count and protein level,
thereby confounding the diagnosis. Diagnostic uncertainty can be decreased by
using accepted corrective formulas. White blood cell differential may be
misleading early in the course of meningitis, because more than 10 percent of
cases with bacterial infection will have an initial lymphocytic predominance
and viral meningitis may initially be dominated by neutrophils. Culture is the
gold standard for determining the causative organism in meningitis. However,
polymerase chain reaction is much faster and more sensitive in some
circumstances. Latex agglutination, with high sensitivity but low specificity,
may have a role in managing partially treated meningitis. To prove herpetic,
cryptococcal, or tubercular infection, special staining techniques or
collection methods may be required.
Primary care physicians frequently perform lumbar
puncture, because cerebrospinal fluid (CSF) is an invaluable diagnostic window
to the central nervous system (CNS). Commonly performed tests on CSF include
protein and glucose levels, cell counts and differential, microscopic
examination, and culture. Additional tests such as opening pressure,
supernatant color, latex agglutination, and polymerase chain reaction also may
be performed. Knowing which tests to order and how to interpret them allows
physicians to use CSF as a key diagnostic tool in a variety of diseases

Supernatant Color

Normal CSF is crystal clear. However, as few as 200 (WBCs) per mm³ or
400 red blood cells (RBCs) per mm³ will cause CSF to appear turbid. Xanthochromia is a
yellow, orange, or pink discoloration of the CSF, most often caused by the
lysis of RBCs resulting in hemoglobin breakdown to oxyhemoglobin, methemoglobin,
and bilirubin. Discoloration begins after RBCs have been in spinal fluid for
about two hours, and remains for two to four weeks.
Xanthochromia is present in more than 90 percent of patients within 12 hours of
subarachnoid hemorrhage onset² and in patients with serum bilirubin levels between
10 to 15 mg per dL (171 to 256.5 µmol per L). CSF protein levels of at least
150 mg per dL (1.5 g per L)--as seen in many infectious and inflammatory
conditions, or as a result of a traumatic tap that contains more than 100,000
RBCs per mm³--also
will result in xanthochromia. Newborn CSF is often xanthochromic because of the
frequent elevation of bilirubin and protein levels in this age group.

Cell Count

Normal
CSF may contain up to 5 WBCs per mm³ in adults and 20 WBCs per mm³ in
newborns. Eighty-seven percent of patients with bacterial
meningitis will have a WBC count higher than 1,000 per mm,³ while 99
percent will have more than 100 per mm³. Having less than 100 WBCs per mm³ is more
common in patients with viral meningitis.
ElevatedWBC counts also may occur after a seizure, in intracerebral
hemorrhage, with malignancy, and in a variety of inflammatory conditions. Table
2 lists common CSF findings in various types of meningitis.

Peripheral blood in the CSF after a "traumatic
tap" will result in an artificial increase in WBCs by one WBC for every
500 to 1,000 RBCs in the CSF. This correction factor is accurate as long as the
peripheral WBC count is not extremely high or low.
A traumatic tap occurs in approximately 20
percent of lumbar punctures. Common practice is to measure cell counts in three
consecutive tubes of CSF. If the number of RBCs is relatively constant, then it
is assumed that the blood is caused by an intracranial hemorrhage. A falling
count is attributed to a traumatic tap. The three-tube method, however, is not
always reliable.
Xanthochromia is a more reliable predictor of
hemorrhage. If a traumatic tap occurs within 12 hours of a suspected
subarachnoid hemorrhage, it is reasonable to repeat the lumbar puncture one
interspace up to try and obtain clear CSF.

Cell Differential
The WBC count seen in normal adult CSF is comprised of approximately 70 percent
lymphocytes and 30 percent monocytes. Occasionally, a solitary eosinophil or
polymorphonucleocyte (PMN) will be seen in normal CSF.² Several
PMNs in a neonatal patient's CSF is not unusual.
The majority of patients with Guillain-Barré syndrome will have 10 or fewer
monocytes per mm³ and a minority of patients will have 11 to 50
monocytes per mm³. Up to 50 monocytes per mm³ are seen
in about 25 percent of patients with multiple sclerosis. The cell
differential alone cannot differentiate between bacterial and nonbacterial
meningitis. Lymphocytosis is seen in viral, fungal, and tuberculous infections
of the CNS, although a predominance of PMNs may be present in the early stages
of these infections. CSF in bacterial meningitis is typically dominated by the
presence of PMNs. However, more than 10 percent of bacterial meningitis cases
will show a lymphocytic predominance, especially early in the clinical course
and when there are fewer than 1,000 WBCs per mm³(Table2).
Eosinophilic meningitis is defined as more than 10 eosinophils per mm³ or
a total CSF cell count made up of more than 10 percent eosinophils. Parasitic
infection should be suspected in this situation. Other possible causes may
include viral, fungal, or rickettsial meningitis; having ventriculoperitoneal
shunts with or without coexisting infection; malignancy; and adverse drug
reactions.

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Microscopic Examination

Gram stain is positive in 60 to 80 percent of untreated cases of bacterial
meningitis and in 40 to 60 percent of partially treated cases. The sensitivity
according to the causative organism ranges from 90 percent in pneumococcal or
staphylococcal meningitis to less than 50 percent in Listeria meningitis.
Hyphae can occasionally be seen in Candida or other fungal meningitis cases.
Several factors influence the sensitivity of Gram
stain. Laboratory techniques used to concentrate and stain CSF can greatly influence
reliability. Cytocentrifugation increases the ability to detect bacteria.Greater numbers of colony-forming units (CFU) per mm³ of CSF increase the likelihood of a positive result. Staining will be positive in 25 percent of cases if fewer than 1,000 CFU per mm³
are present, and in 75 percent of cases if more than 100,000 CFU per mm³ are present.
Lastly, the experience of laboratory personnel is very important. Up to 10 percent of initial
Gram stains are misread.
Acid-fast staining should be done if tuberculosis is clinically suspected. Only 37 percent of initial smears will be positive for acid-fast bacilli. This result can be increased to 87 percent if
four smears are done.Sensitivity also can be increased by examining the CSF sediment.
Other stains should be performed if indicated by the situation. Cryptococcus may be
identified up to 50 percent of the time on an India ink preparation. A tap-water control should always be done to ensure that the India ink is not contaminated.

Toxoplasmosis can be diagnosed with Wright or Giemsa stain. A simple wet preparation of CSF under a cover slip can yield positive results in a variety of protozoan and helminthic infections.
Protein Level
CSF protein concentration is one of the most sensitive indicators of pathology within the CNS. Newborn patients have up to 150 mg per dL (1.5 g per L) of protein. The adult range of 18 to 58 mg per dL (0.18 to 0.58 g per L) is reached between six and 12 months of age.
The physician should know what the normal reference range is for his or her
laboratory, because the measurement is somewhat technique-dependent.
Elevated CSF protein is seen in infections, intracranial hemorrhages, multiple
sclerosis, Guillain Barré syndrome, malignancies, some endocrine abnormalities,
certain medication use, and a variety of inflammatory conditions (Table 3).
Protein concentration is falsely elevated by the presence of RBCs in a
traumatic tap situation. This can be corrected by subtracting 1 mg per dL (0.01 g per L) of protein for every 1,000 RBCs per mm [Evidence level B: observational study] This correction is only accurate if the same tube is used for the protein and cell counts.
Low CSF protein levels can occur in conditions
such as repeated lumbar puncture or a chronic leak, in which CSF is lost at a
higher than normal rate. Low CSF protein levels also are seen in some children
between the ages of six months and two years, in acute water intoxication, and
in a minority of patients with idiopathic intracranial hypertension.

[b]Glucose Level

A true normal range cannot be given for CSF glucose.
As a general rule, CSF glucose is about two thirds of the serum glucose
measured during the preceding two to four hours in a normal adult. This ratio
decreases with increasing serum glucose levels. CSF glucose levels generally do
not go above 300 mg per dL (16.7 mmol per L) regardless of serum levels.
Glucose in the CSF of neonates varies much more than in adults, and the
CSF-to-serum ratio is generally higher than in adults.CSF protein levels do not fall in hypoproteinemia. CNS infections can cause lowered CSF glucose
levels, although glucose levels are usually normal in viral infections (Table2). Normal glucose levels do not rule out infection, because up to 50 percent of patients who have bacterial meningitis will have normal CSF glucose levels.

Chemical meningitis, inflammatory conditions, subarachnoid hemorrhage, and hypoglycemia
also cause hypoglycorrhachia (low glucose level in CSF). Elevated levels of
glucose in the blood is the only cause of having an elevated CSF glucose level.
There is no pathologic process that causes CSF glucose levels to be elevated.

Culture
Cultures done on 5 percent sheep blood agar and enriched chocolate agar remain the gold
standards for diagnosing bacterial meningitis.
Antibiotic treatment prior to lumbar puncture can decrease the sensitivity of
culture, especially when given intravenously or intramuscularly.
Enterovirus, the leading cause of viral meningitis, can be recovered in 40 to 80 percent of
cases. Culture for herpes simplex virus is 80 to 90 percent sensitive but can
take five to seven days to become positive. Results of viral cultures
rarely change the initial management of meningitis.

Mycobacterium tuberculosis is best grown using multiple large volume samples of
CSF. At least 15 mL and preferably 40 to 50 mL of CSF are recommended. Culture
is positive 56 percent of the time on the first sample, and improved to 83
percent oftime if four separate samples are cultured.
These cultures often take up to six weeks for positive identification.
Fungal cultures are positive in more than 95
percent of Cryptococcus neoformans cases and in 66 percent of candidal
meningitis cases. Other fungi are less likely to be culture positive.
Similar to tuberculous meningitis, culture yield in fungal meningitis can be
increased by obtaining large volumes of CSF via repeated lumbar punctures.

Latex Agglutination
Latex agglutination (LA) allows rapid detection of bacterial antigens in CSF.
Sensitivity varies greatly between bacteria. LA for Haemophilus influenzae
has a sensitivity of 60 to 100 percent, but is much lower for other bacteria.
The specificity for LA is very low. However, LA can be useful in partially treated
meningitis cases where cultures may not yield an organism. Because
false positives lead to unnecessary treatment, LA is not routinely used today.
Some experts suggest using LA in cases of suspected bacterial meningitis if the
initial Gram stain and bacterial culture are negative after 48 hours.

Polymerase Chain Reaction
Polymerase chain reaction (PCR) has been a great advance in the diagnosis of meningitis.
PCR has high sensitivity and specificity for many infections of the CNS, is
fast, and can be done with small volumes of CSF. Although testing is expensive,
there is a potential for cost savings by decreasing overall diagnostic testing
and intervention.

PCR has been especially useful in the diagnosis of viral meningitis. PCR of the CSF
has a sensitivity of 95 to 100 percent, and a sensitivity of 100 percent for
herpes simplex virus type 1, Epstein-Barr virus, and enterovirus.PCR is
faster and more sensitive than culture for enterovirus meningitis.²² When PCR
is positive for enterovirus, it allows earlier hospital discharge and less
intervention. [Evidence level B: retrospective chart review]

PCR
is the most sensitive means of diagnosing CMV infections of the CNS, and it
has been suggested that PCR should replace brain biopsy as the gold standard
for herpes encephalitis.

PCR has a sensitivity of 54 to 100 percent and a
specificity of 94 to 100 percent for tuberculous meningitis, and could replace
acid-fast bacillus smear and culture as the test of choice.
PCR is sensitive for acute neurosyphilis but not for more chronic forms.²¹
PCR also is being studied as a diagnostic tool for bacterial meningitis and
other infections of the CNS[/b]

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