Triple phosphate present in urine
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Triple Phosphate (Struvite) Crystals in Urine
What Are They?
Triple phosphate crystals - also called struvite crystals - are composed of magnesium ammonium phosphate (MgNH₄PO₄). The name "triple phosphate" historically refers to the three cations: magnesium, ammonium, and potassium (though the primary composition is Mg-NH₄-PO₄).
Appearance
The hallmark microscopic appearance is the "coffin lid" shape - beveled rectangular prisms with a flat, rectangular face resembling the lid of a coffin. Variants include flower-like and scissors-like structures.
Crystal types compared visually:
- Under polarized light, triple phosphate crystals polarize light strongly
- They precipitate in alkaline urine (pH ≥ 7.0)
Conditions for Formation
Triple phosphate crystals appear almost exclusively in alkaline urine and are strongly associated with:
- Urinary tract infections with urease-producing bacteria - the bacteria break down urea to ammonium (NH₄⁺) and OH⁻, making urine highly alkaline and causing phosphate to precipitate with Mg²⁺ and NH₄⁺.
Key urease-producing organisms include:
- Proteus spp. (most common)
- Klebsiella
- Pseudomonas
- Haemophilus
- Yersinia spp.
- Staphylococcus epidermidis
- Serratia
- Ureaplasma urealyticum (does not grow on routine culture - requires specific culture request)
Note: E. coli - the most common UTI organism - is NOT a urease producer and does not cause struvite formation.
- Urinary stasis - indwelling catheters, neurogenic bladder, genitourinary anomalies, spinal cord lesions.
Clinical Significance: Struvite/Infection Stones
When triple phosphate crystals aggregate into stones, they are called struvite stones (also "infection stones"). Key features:
| Feature | Detail |
|---|---|
| Growth rate | Rapid - grow quickly to large size |
| Stone type | Most staghorn calculi are struvite |
| Can reduce GFR | Yes, in the affected kidney |
| Gender predisposition | Women > men (higher UTI prevalence) |
| Also called | "Stone cancer" due to significant morbidity |
| Composition | Magnesium ammonium phosphate + calcium carbonate apatite |
| Radiograph | Radiopaque (visible on plain X-ray) |
Diagnosis is suggested by: alkaline urine (pH ≥ 7.0) + culture of urease-producing bacteria + large stones.
Urine Color Note
In large quantities, triple phosphate (struvite) crystals can cause white/turbid urine (from calcium phosphate or struvite precipitation) - and rarely a pink/red-brown hue.
Treatment of Struvite Stones
- Complete stone removal is essential - bacteria persist in stone interstices and stones regrow unless fully eradicated
- Small stones (<2 cm): ESWL may suffice
- Larger stones: Percutaneous nephrolithotomy (PCNL), often combined with ESWL
- Antibiotic therapy: Culture-directed; once urine is sterile (~2 weeks post-initiation), dose is halved; monthly cultures for 3 months; continue surveillance for a full year
- Stone fragments should be cultured
Quick Summary Table
| Property | Triple Phosphate (Struvite) |
|---|---|
| Composition | Magnesium ammonium phosphate |
| Urine pH | Alkaline (≥ 7.0) |
| Shape | Coffin lid (beveled rectangular prism) |
| Polarized light | Strongly birefringent |
| Associated with | Urease-producing UTI, urinary stasis |
| Key organism | Proteus spp. |
| Stone type | Staghorn calculi |
- Comprehensive Clinical Nephrology, 7th Ed., p. 69-70
- National Kidney Foundation Primer on Kidney Diseases, 8th Ed., p. 58-59
- Campbell-Walsh-Wein Urology
- Bailey & Love's Short Practice of Surgery, 28th Ed.
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