Urinalysis and dipstick testing - limitations and interpretation
Core concept
- Dipstick protein pad = tetrabromophenol blue, a colorimetric "protein error of indicators" reaction
- Binds albumin almost exclusively
- Blind to non-albumin protein - free light chains (Bence Jones), tubular proteins (beta-2 microglobulin, retinol-binding protein), Tamm-Horsfall
- Blind to microalbuminuria - threshold for a trace is ~150-300 mg/L
- Two false-negative traps and two false-positive traps
- False negative - light-chain proteinuria (myeloma), dilute urine
- False positive - very alkaline urine (pH >8), concentrated urine, chlorhexidine/quaternary ammonium contamination, gross haematuria
- -> A negative dipstick does not exclude proteinuria. If myeloma is possible, order serum free light chains + serum/urine EPG-immunofixation, not a dipstick
3 more sections, plus exam facts
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