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PA22.1-2 | Urine & Body Fluid Analysis — SDL Guide (Part 3)

Pleural and Peritoneal Effusions — Transudate vs Exudate

Effusions accumulate when the balance of hydrostatic/oncotic forces is disrupted (transudate) or when inflammation/infection/malignancy damages the membrane (exudate). The clinical and analytical distinction is essential because management differs entirely.

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Light's criteria (1972, still the gold standard for pleural fluid):
An effusion is an exudate if ANY ONE of these is true:
1. Pleural fluid protein / serum protein > 0.5
2. Pleural fluid LDH / serum LDH > 0.6
3. Pleural fluid LDH > 2/3 upper limit of normal serum LDH

If NONE is met → transudate.

Feature	Transudate	Exudate
Protein	<30 g/L (<3 g/dL)	>30 g/L (>3 g/dL)
LDH	<200 U/L	>200 U/L
Fluid/serum protein ratio	<0.5	>0.5
Appearance	Clear, straw-yellow	Turbid, cloudy, haemorrhagic, chylous
Cell count	<1000/mm³ (mostly mononuclear)	>1000/mm³
Glucose	= serum	Low (TB, malignancy, rheumatoid)
pH	>7.3	<7.2 (empyema, TB, malignancy)

Transudate causes: Heart failure (most common), hypoalbuminaemia (cirrhosis, nephrotic syndrome), hypothyroidism, Meigs' syndrome.
Exudate causes: Infection (parapneumonic/empyema), malignancy, TB, pulmonary embolism, autoimmune (SLE, RA), chylothorax.

Peritoneal fluid (ascites) — SAAG (Serum-Ascites Albumin Gradient): SAAG = serum albumin − ascitic fluid albumin.
• SAAG ≥1.1 g/dL → portal hypertension (cirrhosis, cardiac ascites, Budd-Chiari syndrome)
• SAAG <1.1 g/dL → non-portal causes (malignancy, TB peritonitis, nephrotic syndrome, pancreatitis)

Note: Light's criteria are for pleural fluid. SAAG (not Light's) is the preferred tool for ascites, as it reflects the underlying haemodynamic mechanism rather than membrane permeability.

Empyema: Frank pus. pH <7.0, glucose <2.2 mmol/L, LDH extremely elevated. WBCs predominantly neutrophils (>10,000/mm³). Requires drainage + antibiotics.

Haemorrhagic effusion: Haematocrit fluid/blood >50% → haemothorax. RBC count >100,000/mm³ suggests trauma, malignancy, pulmonary infarction.

CSF Analysis — Normal vs Meningitis Patterns

Cerebrospinal fluid (CSF) is produced by the choroid plexus (~500 mL/day, circulating volume ~150 mL). Normal opening pressure: 7–18 cm H₂O (lateral decubitus). Analysis includes appearance, cell count and differential, protein, and glucose (always with simultaneous serum glucose).

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Normal CSF: Crystal-clear, colourless, WBC <5 (all mononuclear lymphocytes), protein 15–45 mg/dL, glucose 2/3 of serum (50–80 mg/dL, ratio >0.6), no organisms, no xanthochromia.

Xanthochromia (yellow discolouration): due to oxyhaemoglobin (pink, 2–4 hours post-SAH) → bilirubin (yellow, 12 hours–2 weeks). Differentiates subarachnoid haemorrhage from traumatic tap (traumatic tap: no xanthochromia, clears as tubes collected sequentially).

Feature	Bacterial	Viral (aseptic)	Tubercular (TB)	Fungal (Cryptococcus)
Appearance	Turbid, purulent	Clear/slightly turbid	Clear or opalescent, cobweb clot	Clear or opalescent
Pressure	Raised	Normal/mildly raised	Raised	Raised
WBC count	1,000–10,000/mm³	10–1,000/mm³	100–500/mm³	Variable, often <500
Cell type	PMN (neutrophils)	Mononuclear (lymphocytes)	Mononuclear (lymphocytes)	Mononuclear (lymphocytes)
Protein	Markedly raised (>100 mg/dL)	Normal to mildly raised (45–100 mg/dL)	Raised (100–500 mg/dL)	Mildly–moderately raised
Glucose	Very low (<45 mg/dL, ratio <0.4)	Normal (>60 mg/dL)	Low (<45 mg/dL)	Low to normal
Organisms/tests	Gram stain + culture positive	PCR for virus	AFB smear/culture/ADA elevated	India ink +; latex agglutination (CrAg) +

Key differentiating points:
• PMNs in CSF → bacterial until proven otherwise. A viral meningitis can begin with PMNs but shifts to lymphocytes within 24–48 hours.
• TB meningitis: protein is disproportionately high, glucose disproportionately low; a cobweb clot on standing is characteristic; ADA >10 U/L is supportive.
• Cryptococcal meningitis (immunocompromised — HIV CD4 <100): India ink shows the encapsulated yeast ('halo' appearance); latex agglutination for cryptococcal antigen (CrAg) is 95%+ sensitive.
• Glucose interpretation: always use CSF:serum ratio; in hyperglycaemia, CSF glucose may appear 'normal' but the ratio is still low.

Synovial Fluid Analysis

Synovial Fluid Analysis

Synovial fluid (SF) is a transudate of plasma plus lubricin/hyaluronate secreted by synoviocytes. Normal SF: clear, pale yellow, viscous (string sign ≥6 cm), WBC <200/mm³ (<25% PMNs), glucose ≈ serum glucose.

SF is classified by its gross and microscopic appearance:

Class	WBC/mm³	PMN%	Example
Normal	<200	<25%	—
Non-inflammatory	200–2,000	<25%	OA, trauma, CPPD (sometimes)
Inflammatory	2,000–50,000	>50%	RA, gout, pseudogout, reactive arthritis
Septic (infectious)	>50,000	>75%	Septic arthritis (bacterial)
Haemorrhagic	RBCs present	—	Trauma, haemophilia, pigmented villonodular synovitis

Crystal analysis (compensated polarised light microscopy):

• Monosodium urate (MSU) — gout:
• Shape: long needles (may be intracellular in WBCs)
• Birefringence: negatively birefringent → yellow when parallel to the slow axis, blue when perpendicular
• Appearance: 'matchstick' or needle, sharp ends

• Calcium pyrophosphate dihydrate (CPPD) — pseudogout:
• Shape: rhomboid/short rod, often intra- or extracellular
• Birefringence: positively birefringent → blue when parallel, yellow when perpendicular
• X-ray: chondrocalcinosis (calcification of articular cartilage)

Septic arthritis: The most important emergency — joint fluid WBC >50,000 with >75% PMNs and positive Gram stain/culture warrants immediate joint drainage and IV antibiotics. Never miss it; glucocorticoid injection into an infected joint is catastrophic.

Mucin clot test: Acetic acid added to SF — good clot (rope-like) = normal/non-inflammatory; poor/friable clot = inflammatory/septic.