Three numbers carry most of the meaning in a feline chemistry panel: potassium 3.5 to 5.5 mmol/L, sodium 150 to 160 mmol/L and chloride 110 to 125 mmol/L. A cat outside all 3 needs a diagnosis, not a supplement. A cat outside 1 needs a repeat draw, a cause and a recheck date. Almost every abnormal panel sorts into 3 patterns, and each pattern has its own timeline.
The 3 numbers and what moves them
About 98 percent of body potassium sits inside cells, so a serum value describes the small extracellular pool and reacts to acid-base state, insulin and cell damage rather than to total body stores. Sodium sets osmolality and water balance. Chloride follows sodium but tracks vomiting and renal tubular handling more closely. Read all 3 beside creatinine, phosphorus and bicarbonate from the same draw: potassium at 6.0 mmol/L with creatinine at 5.1 mg/dL is a different case from 6.0 with creatinine at 1.4.
Use the interval printed on your own report: analyzers differ, and some laboratories print 147 to 157 for sodium or 107 to 120 for chloride. A value 1 unit outside is worth a conversation, not a panic.
Pattern 1: potassium below 3.5
Chronic kidney disease (CKD) is the commonest cause in cats, and roughly 1 in 4 CKD cats runs low at some point in the disease. Hyperthyroidism hides it: potassium reads normal while the thyroid drives potassium into cells, then drops 2 to 4 weeks after treatment starts. Vomiting, diarrhea, insulin and long diuretic courses do the same. Below 3.0 mmol/L the heart and skeletal muscle both suffer.
What owners notice first is hind-leg weakness after 1 flight of stairs, a neck that droops with the chin toward the chest (cervical ventroflexion), constipation, and a cat that stops grooming. Recheck 7 days after any food or supplement change, and 24 to 48 hours after intravenous fluids are started, because fluids dilute potassium fast. Supplements exist but the amount is weight-based and prescription-only.
Pattern 2: potassium above 5.5
In cats the top cause is urethral obstruction in a male, followed by acute kidney injury, severe acidosis and hypoadrenocorticism, which is rare in cats but easy to miss. Between 5.5 and 6.5 mmol/L most cats show no rhythm change; between 6.5 and 7.5 an ECG is warranted the same day; above 7.5 the risk of a fatal arrhythmia is real and the cat should not wait for morning.
The trap is the rebound. Once a blocked cat is unblocked and starts passing urine, potassium can fall from 8.0 to 3.0 mmol/L inside 6 to 12 hours, so a recheck at 2 to 4 hours and again at 12 hours is standard. A cat sent home after unblocking without a 24-hour potassium check is the classic readmission.
Pattern 3: sodium and chloride drift
Sodium above 160 mmol/L means a water deficit, and the size of it can be estimated from weight: 1 kg of lost body water is about 1 L. Chronic hypernatremia has to come down slowly, no faster than 0.5 mmol/L per hour, because a fast drop swells brain cells and causes seizures. Sodium below 150 with chloride below 110 points to vomiting or dilution; chloride above 125 with a normal sodium points to dehydration or renal tubular acidosis.
Chloride under 110 with a normal sodium is the pattern most often missed: a cat vomiting 3 or more times a week for a month can sit at a chloride of 105 with a normal potassium, and the fix is the vomiting, not salt in the food.
| Pattern | Potassium | Sodium | Chloride | Recheck |
|---|---|---|---|---|
| Low potassium | under 3.5 | 150 to 160 | 110 to 125 | 7 days |
| Severe low potassium | under 3.0 | 150 to 160 | 110 to 125 | 24 to 48 hours |
| High potassium | 5.5 to 6.5 | 150 to 160 | 110 to 125 | 24 hours |
| Obstruction potassium | 6.5 to 8.0 | 145 to 160 | 105 to 120 | 2 to 4 hours |
| Water deficit | 3.5 to 5.5 | over 160 | over 125 | 6 to 12 hours |
| Vomiting loss | under 3.5 | under 150 | under 110 | 48 to 72 hours |
Treat the table as 4 columns read together. A potassium of 3.4 with a chloride of 118 and a sodium of 154 is a kidney story; 3.4 with a chloride of 104 is a gut story, and the second one resolves when the vomiting does.
Why 1 tube can still be wrong
Hemolysis adds potassium, roughly 0.2 to 0.5 mmol/L in a mildly pink sample and more in a red one, and a cat with a difficult draw produces exactly that sample. An EDTA tube gives a potassium above 10 mmol/L and a falsely low calcium. A sample left 6 hours before separation drifts upward by about 0.3 mmol/L. A tight tourniquet for 60 seconds or a 25-gauge needle through a small vein does the same.
A single high potassium in a bright, eating cat is a reason to repeat the draw, not to treat. Repeat within 2 to 4 hours, clean venipuncture, tube filled to the mark. If the second value is normal, the first was an artefact.
Recheck intervals that actually matter
After intravenous fluids start: 24 hours, then 48. After a potassium supplement is prescribed: 48 to 72 hours, then 7 days. After unblocking a male cat: 2 to 4 hours, 12 hours, 24 hours. Stable CKD without supplements: every 3 to 6 months, and always with creatinine and phosphorus on the same panel. A cat on insulin for diabetes: 7 days after any dose change, because insulin pushes potassium into cells along with glucose.
Keep a photo of each report: the trend across 3 panels over 6 months decides whether a kidney cat is stable or sliding.
When it is a same-day visit
- Potassium above 7.0 mmol/L, or a male cat that has produced no urine for 12 hours.
- Potassium under 2.8 mmol/L, or a neck that droops so the chin rests on the chest.
- Sodium above 165 mmol/L, or any seizure or wobbling walk with a high sodium.
- Weakness severe enough that the cat cannot climb into the litter box.
- Vomiting 3 or more times a week for 2 weeks, with weight loss of 200 g or more.
Bring the last 2 or 3 panels and the exact food, including treats. Urine matters too: a cat with a potassium of 3.2 and a urine specific gravity of 1.012 is a kidney case, and a potassium of 3.2 with 1.050 is a gut case. If a urinary infection is suspected, the collection method changes the answer, which is covered at /health/health-cat-urine-culture/. For a cat that has had no vaccines and is going into a cattery, the titer questions sit at /health/health-cat-vaccine-titer/.
If the cat also ate something it should not have, check the ingredient at /tools/toxic/ before you accept a kidney diagnosis; lilies, cholecalciferol baits and grapes all move these same 3 numbers.
Four steps for this week: get the printed reference interval with the result, repeat any single abnormal value within 48 hours, write the recheck date on the calendar, and ask whether the panel included creatinine, phosphorus and a urine specific gravity.
Terms mentioned in this article
Frequently asked
My cat's potassium is 3.4 mmol/L. Is that dangerous?
It is mildly low rather than an emergency, but it should not be ignored. Values between 3.0 and 3.5 usually mean a kidney or gut cause, and the first step is a repeat draw with creatinine and a urine specific gravity on the same visit. Recheck 7 days after any food or supplement change, and sooner if the cat is weak or constipated.
The vet said the high potassium was a lab error. How can that happen?
Hemolysis, an EDTA tube, a 6-hour delay before separation and a difficult venipuncture all push potassium up by 0.2 to 0.5 mmol/L or more. That is why a bright, eating cat with a single high value gets a repeat draw within 2 to 4 hours instead of treatment. Ask for the second sample to be a clean, full tube.
How often should a stable kidney cat have electrolytes checked?
Every 3 to 6 months when the cat is stable and not on potassium supplements, with creatinine and phosphorus on the same panel. After starting fluids, recheck at 24 and 48 hours; after a supplement is prescribed, at 48 to 72 hours and again at 7 days. Keep the printed reports, because the trend across 3 panels matters more than any single number.