PA22.1-2 | Urine & Body Fluid Analysis — Summary & Reflection

REFLECT

You have now covered physical, chemical, and microscopic urinalysis; the five major cast types and their disease associations; urine crystals; transudate/exudate differentiation with Light's criteria and SAAG; the classic four-column CSF comparison table; and synovial fluid crystal analysis.

Before moving on, answer these to yourself:
1. A patient's urine has 3+ protein but NO haematuria and NO casts. Which nephrological syndrome fits? What is the most common cause in a 4-year-old child?
2. You receive a pleural fluid report: protein 22 g/L, serum protein 60 g/L, LDH 150 U/L (serum ULN 180 U/L). Transudate or exudate? What is the single most common cause?
3. CSF shows 8,000 WBCs (90% PMNs), protein 280 mg/dL, glucose 18 mg/dL. What do you do in the next 30 minutes?

If any of these three paused you, return to the relevant section. These are the exact one-liners that appear in SEQ/SAQ examinations.

KEY TAKEAWAYS

Urinalysis — systematic approach:
• Physical → chemical (dipstick) → microscopy (cells → casts → crystals → organisms)
• Dipstick pairs: LE+nitrite = UTI; blood+protein (no glucose) = GN; glucose+ketones = DKA; protein alone = nephrotic
• Dysmorphic RBCs + RBC casts = glomerulonephritis; WBC casts = pyelonephritis; muddy-brown granular = ATN; fatty casts/oval fat bodies = nephrotic
• Crystals: calcium oxalate (envelope, any pH), uric acid (rhomboid, acid), triple phosphate (coffin-lid, alkaline, staghorn stones), cystine (hexagonal, acid, COAL defect)

Effusions:
• Light's criteria (pleural): protein ratio >0.5 OR LDH ratio >0.6 OR LDH >2/3 ULN → exudate
• SAAG ≥1.1 g/dL → portal hypertension ascites; <1.1 → malignancy/TB/nephrotic

CSF meningitis table (anchor this):
• Bacterial: PMNs ↑↑, protein ↑↑, glucose ↓↓, turbid
• Viral: lymphocytes, normal/mild protein ↑, normal glucose, clear
• TB: lymphocytes, protein ↑↑, glucose ↓, cobweb clot, ADA ↑
• Cryptococcal: lymphocytes, India ink +, CrAg +, in HIV

Synovial crystals:
• MSU (gout) = negative birefringence (yellow parallel) = needle = first MTP
• CPPD (pseudogout) = positive birefringence (blue parallel) = rhomboid = knee/wrist

Always interpret findings in clinical context. A number without a story is incomplete pathology.