Cat Blood Test Ranges and When to Recheck

A cat's chemistry panel comes back with a dozen numbers, six of them flagged, and no sentence telling you which flags matter. Reference ranges are local: the same creatinine of 2.0 mg/dL can be normal at one laboratory and high at another, because each lab builds its interval from its own analyzer, its own calibration and its own pool of local cats. What travels between clinics is the pattern. Kidney values that move together, one liver enzyme alone, a glucose that was fear rather than diabetes — that is the part worth acting on.

Use the range printed on your own report

Every number in this article is a common interval, not a law. A typical feline panel reads BUN 16–36 mg/dL, creatinine 0.8–2.4 mg/dL, SDMA under 14 µg/dL, T4 0.8–4.0 µg/dL, ALT 12–130 U/L, glucose 70–150 mg/dL and potassium 3.5–5.8 mEq/L. If your report says 0.9–2.2, that is the interval for that sample on that machine. Comparing a result to another lab's range is how healthy cats get labeled with kidney disease.

BUN, creatinine and SDMA belong in one conversation

BUN is the least specific of the three. It rises with dehydration, with a high-protein meal in the previous 12 hours, with bleeding into the gut, and with any fall in filtration. A BUN of 45 mg/dL in a cat that ate badly and drank little for two days is a hydration story until proven otherwise. Creatinine is steadier but tracks muscle mass: a thin 14-year-old cat can sit at 2.4 mg/dL with half-normal filtration, while a heavy 6 kg tom shows 1.8 with worse kidneys. SDMA is used as the earlier marker because it depends less on muscle, and many labs describe it rising once roughly 25–40% of kidney function is gone. A single SDMA of 16 µg/dL is a reason to repeat the panel in 2–4 weeks with a urine sample, not a diagnosis.

ALT 12–130: one enzyme is not a liver diagnosis

ALT leaks out of damaged liver cells and its half-life in cats is short, measured in hours. A rise to 200 U/L found on a routine screen in a cat that is eating, has normal bile acids and no jaundice is usually repeated in 2–4 weeks rather than treated. A rise above 3 times the upper limit, or any rise with jaundice, weight loss or a palpable liver, moves the timetable to days and adds bile acids, abdominal ultrasound and a conversation about sampling the liver. Steroid treatment, marked dental disease and several days of near-total anorexia can all push ALT without primary liver disease.

T4 0.8–4.0 and the cat who is also sick

Hyperthyroidism usually lands well above 4.0 µg/dL, often between 6 and 12. The trap is the sick cat: any non-thyroidal illness — kidney disease, diabetes, cancer, severe dental pain — can suppress T4 into the normal band or just below it. A cat with a palpable thyroid nodule, weight loss and a T4 of 3.6 deserves a free T4 by equilibrium dialysis, not reassurance.

Glucose 70–150, and why the first number is untrustworthy

Fear raises blood glucose in cats fast; adrenaline can push a resting 90 mg/dL to 250–400 within minutes of a car ride and a stranger in a white coat. One high glucose in an otherwise well cat is not diabetes. The follow-ups are cheap: a fasting repeat after 20 quiet minutes in the carrier, a urinalysis for glucose and ketones, and fructosamine, which reflects the previous 1–2 weeks and stays normal in stress hyperglycemia. Glucose above 300 mg/dL with ketones in the urine, vomiting or poor appetite is same-day veterinary work rather than a recheck.

Potassium 3.5–5.8 changes muscle function

Mild hypokalemia of 3.0–3.4 mEq/L is common in kidney disease, after vomiting and during insulin treatment, and shows as weakness before it shows on any image. Below 3.0 the cat may stop jumping, carry its head low, and in severe cases cannot lift its neck. Potassium above 5.8 needs the sample questioned first, because hemolysis falsely raises it, then a look at urethral obstruction in males, dehydration or Addison's disease.

How far above range before the phone call

Result	Common interval	Recheck	Add to the visit
BUN 40–60 mg/dL	16–36	1–2 weeks	Urine specific gravity
Creatinine 2.5–2.8	0.8–2.4	2 weeks	USG, SDMA, UPC
Creatinine above 2.9	0.8–2.4	3–7 days	IRIS stage, phosphate, blood pressure
SDMA 15–20 µg/dL	under 14	2–4 weeks	USG, UPC
ALT 131–390 U/L	12–130	2–4 weeks	Bile acids if still high
ALT above 390 U/L	12–130	2–5 days	Bile acids, ultrasound
T4 4.1–5.0 µg/dL	0.8–4.0	2–4 weeks	Free T4 by dialysis
Glucose 150–250 mg/dL	70–150	Fasting repeat	Urinalysis, fructosamine
Potassium 3.0–3.4	3.5–5.8	1–2 weeks	Full renal panel

The size of the flag matters less than the direction of the trend. A creatinine that went 1.5, 1.6, 1.9 over eight months is a kidney losing ground even though every value is inside the printed range.

Things that move the numbers before the cat does

An 8–12 hour fast changes glucose and lipids but barely touches creatinine. Hemolysis — a difficult draw, a shaken tube, a sample left 6 hours before spinning — raises potassium and ALT. A tourniquet held for 60 seconds raises potassium and total protein. Subcutaneous fluids given the day before dilute BUN and creatinine and can make a failing kidney look stable. Say all four out loud at the next visit: last meal, last fluid dose, current drugs and how much the cat drank in the past 24 hours.

Bring the history that changes the read

Three items shift interpretation more than any repeat test: a weight trend over 6–12 months, a measured water intake (one 24-hour bowl measurement; above 45 mL/kg/day counts as polydipsia), and a complete drug list including supplements and spot-on products. Breed matters at the edges — Persian and exotic shorthair lines carry polycystic kidney disease, Burmese and Siamese lines carry amyloidosis risk, Maine Coons and Ragdolls have their own heart diseases — so check the breed pages for what to screen. A cat on long-term treatment for feline herpesvirus needs baseline and 3–6 month monitoring bloodwork, covered in our FHV long-term guide. And a toxoplasma titer is a separate question from a routine panel: a positive IgG alone does not mean active disease, as the toxoplasmosis guide explains.

Three steps after a flagged panel

1. Photograph the whole report, including the lab's own reference column, and keep it for the next visit.
2. Repeat in the window the table above gives, not the same afternoon and not six months later.
3. Always pair a kidney value with a urine specific gravity; a concentrated urine changes the meaning of a high creatinine.

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