9.3 Analysis of Cerebrospinal Fluid (CSF), Serous Fluids, and Synovial Fluid

Analysis of Cerebrospinal Fluid (CSF), Serous Fluids, and Synovial Fluid

Quick Answer: The analysis of extravascular body fluids provides critical diagnostic clues for central nervous system infections, cavity effusions, and joint disease. Key concepts include identifying traumatic taps versus subarachnoid hemorrhage in CSF, applying Light's criteria for serous fluids, and differentiating monosodium urate from calcium pyrophosphate crystals in synovial fluid.

Cerebrospinal Fluid (CSF)

Cerebrospinal fluid surrounds the brain and spinal cord, produced by the choroid plexuses and reabsorbed by the arachnoid villi. It is typically collected via lumbar puncture.

Specimen Collection and Tube Distribution

CSF is routinely collected in 3 to 4 sterile tubes, distributed sequentially to specific departments to avoid contamination:

• Tube 1: Chemistry and Serology (frozen or refrigerated). Most prone to blood contamination from the tap.
• Tube 2: Microbiology (kept at room temperature). Avoids skin flora contamination present in tube 1.
• Tube 3: Hematology (cell counts and differentials; refrigerated). Has the lowest probability of containing cells introduced by the puncture.
• Tube 4: Extra tube for specialized testing or backup.

Traumatic Tap vs. Subarachnoid Hemorrhage

Differentiating a bloody CSF due to a traumatic tap from a true subarachnoid hemorrhage (SAH) is a frequent and critical laboratory task.

Note: Xanthochromia requires RBCs to have been in the CSF for at least 2 hours. It is the hallmark of an older hemorrhage.

CSF Meningitis Profiles

Normal CSF has very few WBCs (0-5/uL, mostly lymphocytes/monocytes), a glucose level of approx. 60-70% of the plasma glucose level (usually 50-80 mg/dL), and a low protein concentration (15-45 mg/dL).

Exam Trap: A normal CSF glucose with a lymphocytic pleocytosis strongly suggests viral meningitis, whereas a profoundly low glucose points toward a bacterial or fungal etiology.

Serous Fluids (Pleural, Pericardial, and Peritoneal)

Serous fluids are ultrafiltrates of plasma that lubricate the membranes of the body cavities. An abnormal accumulation of fluid is called an effusion. Effusions are broadly classified as either transudates or exudates.

Transudate vs. Exudate (Light's Criteria)

• Transudate: Caused by a systemic disruption in fluid filtration and reabsorption (e.g., increased hydrostatic pressure in congestive heart failure, or decreased oncotic pressure in hypoproteinemia/cirrhosis). The fluid is clear, pale yellow, and does not clot.
• Exudate: Caused by localized conditions that increase capillary permeability or decrease lymphatic drainage (e.g., infections, malignancies, inflammation). The fluid is often cloudy, purulent, or bloody, and may clot spontaneously.

Light's Criteria definitively identifies an exudate if at least ONE of the following is met:

1. Fluid-to-Serum Protein Ratio: > 0.5
2. Fluid-to-Serum Lactate Dehydrogenase (LDH) Ratio: > 0.6
3. Fluid LDH: > 2/3 the upper limit of normal for serum LDH.

If none of these criteria are met, the effusion is a transudate.

Synovial Fluid

Synovial fluid provides lubrication and nutrients to articulating joints. It contains hyaluronic acid, which makes it highly viscous. Before analysis (like cell counts), very viscous fluid may be treated with hyaluronidase.

Crystal Identification in Joint Disease

Identifying crystals in synovial fluid via compensated polarized light microscopy is essential for diagnosing microcrystalline arthropathies. The compensator places a red background (first-order red) to determine the sign of birefringence.

Mnemonic: "UR-ine is YELLOW when it hits the toilet (parallel)." Urate = Negative = Yellow when parallel. Alternatively, "Gout = Urate = Yellow/Parallel". CPPD is the exact opposite.

Classification of Joint Disorders

Synovial fluid is categorized based on physical and microscopic findings:

1. Noninflammatory: Clear, yellow, high viscosity. WBCs < 1,000/uL (mostly mononuclear). Seen in osteoarthritis.
2. Inflammatory: Cloudy, yellow, low viscosity. WBCs 2,000-75,000/uL (mostly neutrophils). Seen in immunologic disorders (Rheumatoid arthritis, SLE) and crystal-induced gout/pseudogout.
3. Septic: Cloudy/purulent, yellow-green, extremely low viscosity. WBCs > 50,000-100,000/uL (predominantly neutrophils). Seen in bacterial infections. A medical emergency.
4. Hemorrhagic: Red/brown with RBCs. Seen in trauma, tumors, or coagulation factor deficiencies (e.g., hemophilia).

Exam Trap: Synovial fluid from a patient with a prosthetic joint might display cholesterol crystals (flat, notched plates) or corticosteroid crystals (post-injection), which must not be confused with MSU or CPPD.

Seminal Fluid Analysis

Although often performed in andrology or reference settings, seminal fluid appears on the AMT hematology/body-fluid competency list. Key elements:

• Collection: Abstinence typically 2–7 days; deliver to the lab promptly (ideally within 30–60 minutes) and keep near body temperature during transport.
• Liquefaction: Semen normally liquefies within about 30–60 minutes; failure to liquefy can impair analysis and fertility assessment.
• Macroscopic: Volume, viscosity, pH.
• Microscopic: Sperm concentration/count, motility, and morphology (strict criteria vary by guideline edition).
• Fructose testing of seminal plasma can evaluate seminal vesicle function when azoospermia is investigated.

Special Diagnostic Tests on Urine, Feces, and Gastric Contents

Beyond routine UA, the AMT outline lists specialty assays:

• Bence Jones protein: Immunoglobulin light chains in urine (plasma-cell dyscrasias). Strip pads are albumin-biased; use SSA and immunofixation/electrophoresis for detection/typing.
• Watson-Schwartz test: Differentiates urobilinogen vs. porphobilinogen (PBG) using Ehrlich reagent and solubility in chloroform/butanol—classically used when acute porphyria is suspected.
• Hoesch test: Rapid screen for porphobilinogen in urine.
• Urine/stool/gastric occult blood: Detects occult hemoglobin (guaiac or immunochemical methods). Dietary/peroxidase interferences matter for guaiac tests; fecal immunochemical tests (FIT) are more globin-specific.
• Urine myoglobin vs. hemoglobin: Both may cause a positive blood pad; clinical context, plasma color, and CK help distinguish rhabdomyolysis (myoglobin) from hemolysis.
• Urine pregnancy tests: Detect hCG; interpret false negatives (dilute urine, very early pregnancy, hook effect) carefully.