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Urine Triple Phosphate (Struvite) Crystals

Whether colorless, coffin-lid-shaped struvite crystals show up in a urine sample. They can be harmless, but are linked to alkaline urine and some urinary infections.

AI-verified against cited sources. Awaiting expert reviewLOINC 5814-9

What it is

Triple phosphate crystals, also called struvite or magnesium ammonium phosphate crystals, are tiny crystals a lab can see under the microscope. They are colorless and look like a coffin lid. They tend to form when urine is alkaline, which means it has a higher pH. Labs report them as present or absent, as a number per high-power field (HPF), or in words such as few or many.

Why it matters

A few triple phosphate crystals can be normal. Their main value is as a clue. Larger numbers, especially with signs of infection, can go along with a urine infection from bacteria that make the urine alkaline, and with infection-related kidney stones. Your clinician reads this finding with your symptoms, urine pH, bacteria and white cells, and may order a urine culture. Crystals alone do not mean you have a stone.

If it’s high

More triple phosphate crystals than usual most often reflect alkaline urine. This can come from diet, from a sample that stood before testing, or from a urinary infection with bacteria that make urine alkaline. It is a clue rather than a diagnosis.

Common causes

  • Alkaline urine, for example from a diet high in citrate-rich foods
  • A sample that cooled or stood before it was tested
  • A urinary tract infection with bacteria that split urea, often Proteus
  • Infection-related (struvite) kidney stones, which are less common

What can help

  • Give a fresh clean-catch sample and get it to the lab promptly
  • If you have urinary symptoms, ask whether a urine culture is needed
  • Ask whether your urine pH and the rest of the urinalysis fit together

What can affect your result

  • Urine pH: alkaline urine favors these crystals
  • Diet and fluids
  • A urinary infection with urea-splitting bacteria
  • How fresh the sample is and how it was stored
  • How your lab reports crystals
  • Crystals can form in normal urine as it cools, so a sample that sat or was refrigerated before testing can show crystals that were not there when you gave it (AFP 2022).
  • Bacterial growth or contamination can push urine into the alkaline range, which favors phosphate crystals.
  • Diet affects urine pH: a high-citrate diet can make it more alkaline, and a high-protein diet more acidic (AFP 2022).
  • Labs report crystals differently: present or absent, per high-power or low-power field, or in words.

A fresh midstream clean-catch sample. Crystals can form as urine cools or stands, so labs prefer fresh samples. Reflects the sample on the day. Crystals can appear or disappear as urine pH and concentration change through the day.

When to talk to a clinician

See a clinician soon for burning with urination, fever or chills, back or side pain, or blood in your urine, whatever this finding says. Severe side pain with vomiting needs urgent care.

Questions you could ask

  • Does this finding matter given any kidney stone history I have?
  • Do my urine pH and the rest of the urinalysis point to an infection?
  • Do I need a urine culture, or a repeat urinalysis with a fresh sample?

Questions people ask

Do triple phosphate crystals mean I have a kidney stone?

No. Crystals can show up in healthy people, and they can form in urine that has cooled or stood. A stone is found with imaging and other tests, and your clinician looks at crystals alongside your symptoms and other urine results.

Why are these crystals linked to urinary infections?

Some bacteria make urine more alkaline, and in alkaline urine phosphate and magnesium can form crystals. That is why your clinician may check urine pH, bacteria and sometimes a urine culture.

Show the science and sources

For clinicians and researchers

Struvite is magnesium ammonium phosphate hexahydrate. AFP 2005 describes triple phosphate crystals as sometimes normal but often associated with alkaline urine and urinary infection (typically Proteus), colorless with a "coffin lid" appearance, and notes cloudy urine from precipitated phosphate crystals in alkaline urine. AFP 2022 adds that monohydrated calcium oxalate, uric acid and triple phosphate crystals are birefringent under polarized light, that crystals can form when normal urine cools so only fresh samples should be examined, and that a high urinary pH occurs with infection by urease-positive bacteria and with infected struvite or staghorn stones. Ng 2021 explains the mechanism: urease-producing bacteria (Proteus, Providencia, Morganella, Klebsiella, Pseudomonas, Ureaplasma urealyticum, Corynebacterium urealyticum and others) hydrolyze urea to ammonium and hydroxide, producing persistently alkaline urine that, with abundant phosphate and magnesium, favors crystallization of magnesium ammonium phosphate and large branched stones. The Brazilian nephrolithiasis guideline lists infection-related (struvite) stones among high-risk stone types and notes that alkaline pH in sediment may reflect bacterial contamination. No standard reference interval or grading exists; LOINC offers presence, per-HPF and per-LPF codes, so labs vary.

LOINC 5814-9 (primary), 46137-6, 33020-9, 99900-3. Specimen: urine.

Sources

  1. Urinalysis: A Comprehensive Review, American Family Physician (2005)
  2. Office-Based Urinalysis: A Comprehensive Review, American Family Physician (2022)
  3. Medical evaluation and pharmacotherapeutical strategies in management of urolithiasis, Therapeutic Advances in Urology (2021)
  4. Brazilian Guidelines on evaluation and clinical management of Nephrolithiasis: Brazilian Society of Nephrology, Jornal Brasileiro de Nefrologia (2025)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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