Struvite crystals in urine of cats and dogs
Struvite is the most common crystal type in the urine of dogs and cats, and it is often seen in healthy animals. Crystals are microscopic and are not the same as stones. What a finding means depends on the species, how fresh the sample was, urine pH, and whether there are bacteria or urinary signs.
What it shows
Struvite (magnesium ammonium phosphate) crystals are large colourless prisms, often compared to “coffin lids”, seen in the urine sediment under the microscope. Crystals appear when urine is supersaturated with the minerals they are made of, although even supersaturated urine does not always form them. Struvite can be found at any pH but forms more readily in neutral to alkaline urine.
Crystals are not stones. Crystalluria is not the same as urolithiasis and is not necessarily a sign of disease; on the other hand, stones can be present without any crystals in the urine. Only laboratory analysis of a stone shows its composition for certain.
Higher than reference — what it may be a sign of
- Nothing at all — struvite crystals are often found in clinically healthy dogs and cats.
- Crystals that formed in the sample — after collection, urine cools, stands and evaporates, and its pH can change; crystals then appear that were not there in the bladder. Merck advises examining urine within 30 minutes; it can be refrigerated for up to 24 hours, but cooling itself may make crystals precipitate.
- Urinary tract infection — bacteria that produce urease, an enzyme that turns urea into ammonia, make urine more alkaline and favour struvite crystals and stones. In dogs this is the link that matters (see “Dog” below).
- Struvite stones or urethral plugs in cats — cats can form struvite stones without infection, and struvite is the most common mineral in the soft plugs that can block the urethra.
- Very concentrated or alkaline urine for other reasons — see urine pH.
Lower than reference
No struvite crystals is a common and expected result. It does not rule out a stone: stones can be present without crystals in the urine, so if a stone is suspected the vet relies on imaging, not on the sediment.
Look at together with
- Urine pH — struvite favours neutral to alkaline urine.
- Urine specific gravity — concentrated urine favours crystals.
- Bacteria in urine and urine culture — especially important in dogs.
- Urine blood — bleeding often accompanies stones and inflammation of the bladder.
- Calcium oxalate crystals — the other common crystal type in cats and dogs.
Units and conversion
Crystals are judged by eye under the microscope, so the result is semi-quantitative and has no unit to convert. Crystals are graded subjectively as few, moderate or many; large crystals such as struvite are assessed at low magnification. Some forms give a number or a range per high-power field (/hpf), others only words such as “not found”, “few” or “many”. The app keeps the entry exactly as printed on the form. Results from different labs, or read at different magnifications, can be compared only roughly.
Reference range
The reference range is on your lab’s form; for crystals there is often no number at all, and a few struvite crystals in a dog or cat are not abnormal in themselves. The vet reads the finding together with how fresh the sample was, urine pH and concentration, bacteria and the animal’s symptoms. Crystals that are numerous, persist in fresh samples or come with urinary signs carry more weight than a single finding in urine that stood or was chilled; the vet may ask for a fresh sample to check.
Cats and dogs
In cats struvite matters more than in dogs. Cats can form struvite stones without infection, so struvite crystals in cats may be significant. Crystals without stones have been linked to lower urinary tract signs in some cats, and struvite is the most common mineral in urethral plugs; male cats, whose urethra is longer and narrower, are at greater risk of blockage. Struvite and calcium oxalate are the two most common stone types in cats.
In dogs struvite crystals are not a problem unless there is a urinary tract infection at the same time; without infection they are not linked to struvite stones. In most dogs struvite stones form together with an infection by urease-producing bacteria such as Staphylococcus or Proteus, and female dogs get these infections and stones much more often than males. Sterile struvite stones are rare in dogs.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University) — Urinalysis: crystals
- eClinPath (Cornell University) — Crystal quick guide
- Whitbread TJ. Urinalysis. Merck Veterinary Manual
- Merck Veterinary Manual — Urolithiasis in cats
- Merck Veterinary Manual — Urolithiasis in dogs
- VCA Animal Hospitals — Struvite bladder stones in dogs
- Cornell Feline Health Center — Feline lower urinary tract disease
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.