Two stone types account for nearly all feline bladder stones: struvite (magnesium ammonium phosphate) and calcium oxalate. The practical difference is a 14 to 28 day dissolution window for struvite and none at all for oxalate, so the first urinalysis and the first radiograph decide whether the cat goes on a prescription diet or to surgery. Size, urine pH and a culture matter more than the stone's name on the report.
The two chemistry families
Struvite grows in urine sitting at pH 7.0 or above, usually with a urease-positive infection turning urea into ammonia, and its crystals are coffin-shaped under the microscope. Calcium oxalate grows in acidic urine around pH 5.5 to 6.5, has no dissolving diet, and is the type that blocks a male urethra with a stone of 1 to 2 mm. A third group, urate and cystine, covers fewer than 5 in 100 stones and needs a separate workup, including a liver screen for urate.
Imaging: radiograph first, ultrasound second
Most struvite and oxalate stones are radiopaque, so a survey radiograph with 2 views (lateral and ventrodorsal) is the first test. Read it at full size: a 1 mm stone can hide behind the pelvis or inside the urethra, and a full bladder of 60 to 80 ml can swallow a 3 mm stone. Ultrasound adds the non-radiopaque stones, measures bladder wall thickness (over 2 mm suggests cystitis) and shows sludge; it is also the only practical way to watch a stone move into the urethra. If a stone sits at the trigone, contrast cystography settles it in 1 visit.
Urine: pH, crystals and culture
Collect by cystocentesis when possible, or free-catch and read within 30 minutes, because pH drifts upward as carbon dioxide escapes and a sample left on the bench for 2 hours can read 0.5 units high. Struvite case: pH 7.2 to 8.0, triple phosphate crystals, often bacteria and white cells. Oxalate case: pH 5.5 to 6.5, calcium oxalate dihydrate crystals, usually sterile. Culture every stone cat, because roughly 3 in 10 struvite cases carry an infection that will rebuild the stone within weeks if it is not treated.
Dissolution has 2 fixed imaging points
Struvite under about 10 mm of total burden, in a cat that is passing urine, can be dissolved with a prescription dissolution diet by veterinary prescription; pain control and, when the culture is positive, an antibiotic chosen by sensitivity, again strictly by prescription. Image at day 14 and day 28. A stone that has not shrunk by 30 to 50 percent at day 14 means the diagnosis is wrong, the cat is eating the other cat's food, or the infection is uncontrolled. Dissolution ends when 2 consecutive radiographs show no stone, and the diet continues another 4 weeks, because the bladder is still full of crystal seeds.
Oxalate means a procedure, not a diet
Calcium oxalate cannot be dissolved, so the options are surgical removal by a veterinarian, or, for stones under 5 mm in a female, voiding hydropulsion under sedation. A male cat with a 2 mm stone at the urethral tip is an emergency, not a waiting room case. Submit every removed stone for analysis: the report is the only way to know which diet to run for the next 12 months, and guessing wrong doubles the recurrence risk.
| Item | Struvite | Calcium oxalate |
|---|---|---|
| Usual urine pH | 7.2 to 8.0 | 5.5 to 6.5 |
| Dissolution window | 14 to 28 days | 0 days |
| Typical size at diagnosis | 2 to 12 mm | 1 to 8 mm |
| First imaging recheck | day 14 | week 4 after surgery |
| Then | day 28 | month 3 |
| Long-term imaging | every 6 months | every 3 to 6 months |
The 12-month recurrence map
Recurrence clusters in the first 6 to 12 months, which is why the calendar is front-loaded: imaging at 1 month, 3 months, 6 months, then every 6 to 12 months for life. Water intake is the lever that works on both types; aim for 40 to 60 ml per kg per day, which for a 4.5 kg cat means 180 to 270 ml, delivered by 3 bowls, a fountain, or wet food at 2 meals a day. Keep body condition at 5 out of 9: obesity raises urine concentration and roughly doubles stone risk. For oxalate cats, avoid over-acidifying urine below pH 6.0, since that is the zone where oxalate forms.
What counts as an emergency
A male cat that has produced no urine for 12 hours, or that strains in the box 10 times in an hour with nothing coming out, goes in within the next 60 minutes. A blocked cat can be dead within 24 to 36 hours from potassium rise and bladder rupture, and the first 6 hours are the difference between a catheter and a major surgery. Signs that mean minutes, not hours: collapse, a heart rate under 120, an irregular rhythm, vomiting, or a hard painful bladder the size of a plum. Blood in the litter with normal urine flow can wait for the morning; no urine at all cannot.
What to do this week
- Ask for 2-view radiographs and a urinalysis with pH and culture on the same visit, not on 2 separate visits.
- Book the day 14 image before you start the dissolution diet, so it cannot slide to day 25.
- Demand stone analysis after any surgery, and file the result where you keep the vaccine card.
- Measure water intake for 3 days by weighing the bowls, then add a fountain if the cat is under 40 ml per kg.
- Put the 1, 3, 6 and 12 month imaging dates in the calendar the day the cat comes home.
Budget the follow-up imaging before it arrives; the price of radiographs, ultrasound and urine culture sits in the cost tool. If the urine sample was hard to get, the collection options in fecal test types explain why cystocentesis beats a litter box sample, and a cat that also drinks and urinates heavily should be screened with the curve in insulin types before the bladder is blamed.
Terms mentioned in this article
Frequently asked
Can all cat bladder stones be dissolved with diet?
No. Struvite stones under roughly 10 mm can usually be dissolved in 14 to 28 days with a prescription diet, with imaging at day 14 and day 28. Calcium oxalate stones have no dissolution protocol at all and need surgical removal or, in some females, voiding hydropulsion.
How often should a cat with a stone history be rechecked?
Imaging at 1 month, 3 months and 6 months after the stone is gone, then every 6 to 12 months for life, because most recurrences land inside the first year. Urine pH and a culture should ride along with the 3 month visit, not the annual one.
When is a blocked male cat an emergency?
When no urine has passed for 12 hours, or the cat strains repeatedly with nothing coming out, it needs care within the hour. A complete blockage can kill within 24 to 36 hours through potassium rise and bladder rupture, and the first 6 hours decide how simple the fix is.