Urinalysis and Body Fluid Analysis: Chemical, Microscopic and Fluid Interpretation
Urinalysis and body fluid questions test the correlation between the dipstick, the microscope and the clinical picture. Expect to interpret transudate versus exudate, identify crystals, and explain why a dipstick result might not match the sediment.
Core rules
- Specimen: a first-morning midstream clean-catch specimen is preferred because it is concentrated; samples should be examined within 2 hours at room temperature or refrigerated and brought to room temperature before testing, since cold urine precipitates amorphous urates and phosphates and increases specific gravity.
- Dipstick protein mainly detects albumin; it is relatively insensitive to globulins and Bence Jones protein, so a negative dipstick does not exclude myeloma. Blood reagent detects haemoglobin and myoglobin peroxidase activity, so a positive blood with no red cells suggests haemoglobinuria or myoglobinuria.
- Nitrite is produced by nitrate-reducing gram-negative organisms and is specific but insensitive; leukocyte esterase detects neutrophils and is false negative with lymphocytes.
- Microscopic elements: hyaline casts are non-pathological; red cell casts indicate glomerulonephritis; waxy casts suggest chronic renal failure; renal tubular epithelial cells indicate tubular injury.
- Serous fluid Light criteria for an exudate: pleural protein/serum protein greater than 0.5, pleural LDH/serum LDH greater than 0.6, or pleural LDH greater than two-thirds the upper limit of normal serum LDH. Any one criterion classifies the fluid as an exudate.
Worked example
Pleural fluid from a patient with a large effusion: pleural protein 42 g/L, serum protein 70 g/L, pleural LDH 320 U/L, serum LDH 200 U/L. Reasoning: the protein ratio is 42/70 = 0.60, which exceeds 0.5, and the LDH ratio is 320/200 = 1.60, far exceeding 0.6. Both criteria are met, so the fluid is an exudate, and the differential shifts from heart failure and cirrhosis toward infection, malignancy or pulmonary embolism. Note that diuretic-treated heart failure can produce a pseudoexudate, so the serum-effusion albumin gradient (greater than 12 g/L favours a transudate) is a useful tiebreaker.
Traps that cost marks
- Synovial crystals: monosodium urate is needle-shaped and negatively birefringent (gout); calcium pyrophosphate is rhomboid and weakly positively birefringent (pseudogout).
- Reading a urine specific gravity of 1.000 after a delay, when bacteria have broken down glucose and urea.
- Interpreting a positive nitrite in a specimen that sat at room temperature for hours.
- Forgetting that a traumatic tap raises the CSF cell count; subtract roughly 1 white cell per 700 red cells per microlitre.