CSF And Serous Fluids

Cerebrospinal Fluid: Collection, Counts, And Chemistry

Cerebrospinal fluid (CSF) is obtained by lumbar puncture and collected in three to four sterile tubes. The standard distribution order is tested often: tube 1 to chemistry and serology, tube 2 to microbiology, tube 3 to hematology (cell count and differential), and tube 4 (when drawn) for additional or cytology testing. Using the last tube for cell counts minimizes skin/blood contamination from the puncture.

Normal CSF is crystal clear and colorless; turbidity above ~200 WBC/uL or 400 RBC/uL produces visible cloudiness. The WBC count is performed undiluted in a hemocytometer because normal counts are very low: adults 0-5 WBC/uL (mostly lymphocytes and monocytes), neonates up to ~30/uL. A neutrophilic pleocytosis points to bacterial meningitis; a lymphocytic pleocytosis points to viral or tubercular/fungal meningitis.

CSF chemistry correlations:

Because CSF glucose tracks serum, always draw a paired serum glucose 30-60 minutes before the tap. A low CSF-to-serum glucose ratio with high protein and neutrophils is the classic bacterial-meningitis triad. Oligoclonal bands on protein electrophoresis (present in CSF but not serum) support multiple sclerosis.

Traumatic Tap, Xanthochromia, And Serous Fluids

A bloody CSF poses the central exam question: traumatic tap or subarachnoid hemorrhage (SAH)? Two findings favor a traumatic tap: (1) progressive clearing of blood from tube 1 to tube 4, and (2) a clear, colorless supernatant after centrifugation. Two findings favor true SAH: (1) even blood distribution across all tubes, and (2) xanthochromia, a pink-to-yellow tint of the centrifuged supernatant from RBC lysis and hemoglobin breakdown (oxyhemoglobin, then bilirubin). Xanthochromia takes several hours to develop, so it is absent immediately after a traumatic tap but present in SAH that occurred earlier.

Web-verified: xanthochromia from oxyhemoglobin/bilirubin can take up to ~12 hours to fully develop, and fewer than five RBCs in the final tube argues strongly against SAH.

Serous fluids line the pleural, pericardial, and peritoneal cavities. The first decision is transudate versus exudate:

• Transudate = systemic imbalance (congestive heart failure, hypoalbuminemia, cirrhosis). Clear, low protein, low cell count, low LDH.
• Exudate = local inflammation, infection, or malignancy. Cloudy, high protein, high LDH, high cell count.

Light's criteria define an exudate if ANY one is met:

For peritoneal (ascitic) fluid, the serum-ascites albumin gradient (SAAG) is more useful: SAAG >= 1.1 g/dL indicates portal hypertension (transudate-like), <1.1 g/dL indicates exudate. A markedly low pleural fluid glucose suggests empyema, rheumatoid effusion, or malignancy. Always pair fluid cytology with these chemistries when a malignant effusion is suspected.

Fluid-Specific Counts, Color Clues, And Reporting Nuances

Each serous and special fluid carries its own vocabulary that the BOC tests directly. Pleural fluid that is grossly bloody (hemothorax) is confirmed by comparing the fluid hematocrit to blood: a fluid-to-blood hematocrit ratio above 0.5 defines hemothorax rather than a hemorrhagic effusion. Milky pleural fluid suggests a chylous effusion (thoracic-duct leak, high triglycerides >110 mg/dL with chylomicrons) versus a pseudochylous effusion (chronic, high cholesterol, cholesterol crystals).

Peritoneal lavage counts guide trauma decisions: >100,000 RBC/uL after blunt abdominal trauma indicates significant intra-abdominal bleeding. Synovial fluid is graded by appearance and WBC count:

Synovial fluid viscosity reflects hyaluronate; a positive string test (a string >4 cm) indicates good viscosity, while a watery, low-viscosity fluid suggests inflammation. The mucin clot (Ropes) test with dilute acetic acid forms a tight clot in normal fluid and a friable clot in inflammatory fluid.

Reporting nuances the exam favors: CSF appearance terms include clear, hazy, turbid, bloody, and xanthochromic, and the pellicle/clot that forms in tubercular meningitis is a classic finding. Always centrifuge bloody CSF immediately because delayed processing allows in-vitro hemolysis that mimics xanthochromia and falsely suggests SAH. For amniotic fluid, fetal lung maturity historically used the lecithin-to-sphingomyelin (L/S) ratio, where >=2.0 indicates maturity.

Anchor your studying to logistics: this domain (5-10%) sits within a 100-question, 2.5-hour computer-adaptive exam scaled 100-999 with a 400 passing score, so mastering these fluid-specific discriminators yields disproportionate scored points. A final reporting nuance: CSF and serous-fluid differentials should be performed on a cytocentrifuged, stained smear rather than estimated from the counting chamber, because morphology (neutrophils versus lymphocytes versus macrophages versus malignant cells) drives the clinical interpretation more than the raw total count, and the cytospin concentrates scarce cells for accurate classification.