Filtration, concentration, and protein handling, read together.
The kidneys do several jobs at once — filtering waste from the blood, concentrating urine to conserve water, and holding protein back where it belongs — and no single number captures all of them. That is why kidney results are read as a set.
SDMA and creatinine report on filtration, with SDMA rising earlier and creatinine confirming and staging. Urea (BUN) adds context but is swayed by hydration and diet. Urine specific gravity reports on concentrating ability — a different job that fails early — so a urine sample transforms the meaning of the blood markers. UPC quantifies protein leaking into the urine, one of the strongest predictors of progression. And phosphorus climbs as disease advances and is a central treatment target.
Read together, and — crucially — tracked over time, these markers turn a snapshot into a trajectory. Chronic kidney disease is a slope, not an event, and catching a downward trend early is what lets treatment do the most good. This is especially relevant in older cats, in whom CKD is common and creatinine alone can lag behind the true state of the kidneys.
SDMA is a blood marker of how well the kidneys are filtering. It rises earlier than creatinine — often while a pet still looks completely well — so a value creeping up over successive tests, even one still inside the reference range, is the signal worth watching rather than any single reading.
Creatinine is a muscle waste product cleared by the kidneys, so a raised level points to reduced kidney filtration. It's a late signal — it only climbs once a large share of kidney function is already lost — so it's read alongside SDMA and urine concentration, and the trend over time matters more than a single figure.
Urea is a protein waste product the liver makes and the kidneys clear, so a raised level can reflect reduced kidney filtration. But it's easily swayed by hydration, diet and gut bleeding, so it's interpreted alongside creatinine and SDMA rather than on its own.
Phosphorus is a mineral balanced by the kidneys, gut and bone. It tends to rise as kidney disease advances, and keeping it in the lower part of the range is a central goal of kidney care — so a climbing phosphorus in a kidney patient is an important, actionable signal.
Urine specific gravity measures how concentrated the urine is — how well the kidneys are holding on to water. Dilute urine alongside rising blood kidney markers is far more meaningful than either finding alone, which is why a urine sample is essential to interpreting kidney results.
The UPC ratio measures how much protein is leaking into the urine, corrected for how concentrated that urine is. Persistent proteinuria is one of the strongest predictors that kidney disease will progress, so a UPC that stays above the normal range — or drifts upward over repeated samples — is worth acting on early, often before other kidney markers move.
Potassium is held within tight limits because both high and low levels directly affect the heart and muscles. The kidneys and adrenal glands are its main regulators, so an abnormal value points there — and a markedly high potassium, in particular, can be a medical emergency.
Sodium tracks the balance between salt and water in the body — so it's really a marker of water balance as much as salt. It shifts with hydration, kidney handling and some hormonal conditions, and both high and low values are read in that context rather than as 'too much' or 'too little' salt.
Chloride moves largely in step with sodium and helps describe the body's acid-base balance, so it's interpreted alongside the other electrolytes rather than alone. Its main independent value is in acid-base assessment, where a chloride out of step with sodium is the informative finding.
The sodium-to-potassium ratio is a derived value that acts as a screening flag: a low ratio (classically under 27) is the recognised prompt to consider Addison's disease. It's a red flag to investigate, not a target to optimise.
FGF-23 is a phosphate-regulating hormone that often rises before creatinine in feline kidney disease. IRIS guidance uses it to decide which cats with early CKD benefit from dietary phosphate restriction, so a value climbing toward the upper limit — even before creatinine moves — is worth discussing with your vet.
This derived ratio compares two kidney markers that behave slightly differently, which can help suggest whether a raised urea is coming from dehydration, a gut bleed, or the kidney itself. It's a soft supporting clue in pets — much less reliable than in human medicine — and never a diagnosis.
Calcium and phosphorus are regulated together, so they are read as a pair. In kidney disease the calcium × phosphorus product matters because when both run high, minerals can deposit in soft tissues — including the kidneys themselves — which is a key reason phosphate control is so central to CKD care.
Bicarbonate is the body's main buffer against acid, so total CO₂ on a chemistry panel is essentially a measure of acid-base balance. A low value usually means metabolic acidosis — acid accumulating or bicarbonate being lost — which is common in kidney disease, diabetes and severe diarrhoea.
The anion gap is a calculated value that estimates the 'unmeasured' acids in blood. Its main job is to sort metabolic acidosis into two categories — a high gap means acid has been added (ketones, lactate, toxins), a normal gap means bicarbonate has been lost — which points to very different causes.
A urinalysis is three tests in one — concentration, a chemical dipstick, and microscopic sediment — and it transforms the meaning of blood results. Without a urine sample, kidney blood markers can only ever be half-interpreted, which is why a urinalysis is the most under-requested test in veterinary medicine.
Klotho is a protective protein, made largely by the kidneys, that links kidney function with inflammation, bone and musculoskeletal health — and lower levels track chronic kidney disease and ageing. It is a research assay with no routine clinical reference range in dogs or cats, so it is surfaced as an emerging signal only.
Cystatin C is a small protein filtered freely by the kidneys, so it corroborates early kidney decline alongside SDMA and creatinine — and, unlike creatinine, it is not swayed by muscle mass. Canine and feline reference intervals remain lab-dependent, so it is read as a corroborating trend rather than against a fixed threshold.