41. Laboratory tests of the cerebrospinal fluid and other body fluids.

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Cerebrospinal fluid

Lumbar puncture (LP) is the procedure in which one can obtain and analyse cerebrospinal fluid. Cerebrospinal fluid can be assessed in the evaluation of many neurological conditions. The CSF pressure can also be measured during lumbar puncture.

Indications

Interpretation

Compared to plasma, the CSF has less protein and very few cells. The electrolyte composition is also different, but this isn't of clinical relevance.

Parameter Normal range Bacterial meningitis Viral meningitis Multiple sclerosis
Pressure 8 – 20 cmH2O Elevated Normal Normal
Appearance Clear, like water Turbid Clear Clear
RBCs A few Normal Normal Normal
WBCs < 5 cells/µL Elevated (mostly granulocytes) Elevated (mostly lymphocytes) Elevated (mostly lymphocytes)
Protein < 0,5 g/L Elevated Normal Normal
Glucose > 60-70% of blood glucose Decreased Normal Normal
Oligoclonal bands (on electrophoresis) Absent Absent Absent Present

Microbiological examinations (culture, PCR), immunoglobulins, as well as measurement of certain biomarkers, like those of dementia or prion disease, can also be measured in CSF.

Pleural fluid

Pleural effusion, also called hydrothorax, refers to the presence of fluid in the pleural cavity. If visible on a chest radiograph, there are at least 100 mL of fluid.

Etiology and types

Diagnosis and evaluation

A lot can be gathered from the gross appearance of the fluid alone. Turbid fluid is suggestive of empyema or infection. Bloody fluid is suggestive of haemothorax. Milky fluid is suggestive of chylothorax.

Laboratory evaluation

If the type of pleural fluid is uncertain, it should be drained with a needle for laboratory examination. If the fluid needs to be drained anyway (due to symptoms), the fluid should also be sent for laboratory examination. Only a laboratory evaluation can distinguish a transudate from an exudate.

One can analyse "regular" biochemical tests as well as microbiological tests like culture or PCR and others. Routine tests performed on pleural fluid include white blood cell count with differential, protein, LDH, and glucose. If there is an elevated WBC and it's predominantly polymorphonuclear cells, infection is most likely. If it's predominantly lymphocytes, tuberculosis and lymphoma are more likely.

If malignancy is suspected, one should send the fluid to cytology as well. If infection is suspected, Gram stain and culture should be performed.

Distinguishing between exudate and transudate

There are several approaches to distinguishing between pleural exudate and transudate. Here are two options:

UpToDate recommends using the PFO3 (pleural fluid-only three-test). This is the most simple as it does not require a concomitant blood test. According to the PFO3 criteria, the fluid is an exudate if at least one of the following is fulfilled:

  • The protein concentration of the pleural fluid is > 30 g/L
  • The cholesterol concentration of the pleural fluid is > 1.4 mmol/L
  • The LDH concentration of the pleural fluid is > 67% of the upper normal limit of serum LDH

We can also use Light's criteria to distinguish between transudates and exudates. According to the criteria, the fluid is an exudate if at least one of the following is fulfilled:

  • The protein concentration of the pleural fluid is > 50% of plasma protein concentration
  • The LDH concentration of the pleural fluid is > 60% of serum LDH concentration
  • The LDH concentration of the pleural fluid is > 67% of the upper normal limit of serum LDH

A pleural fluid pH of < 7.30 means an empyema is most likely. If the pH is 7.30 - 7.45, an exudate is more likely, and if 7.40 - 7.55, transudate is more likely. However, the pH alone is not as accurate as using any of the above criteria.