Oliguria is defined as the production of abnormally small amounts of urine. Clinically, it is classified as urine output of approximately 0.1 to 0.2 milliliters per pound of body weight per hour. It sits in the dangerous middle ground between normal urine production and Anuria (zero urine).
In the context of Chronic Kidney Disease (CKD), Oliguria is a terrifying turning point. It almost always signifies that the kidneys have moved from a state of "failing" to "failure."
The "Paradigm Shift" (From Polyuria to Oliguria) For most of their illness, CKD cats are Polyuric. They produce massive amounts of dilute urine because their kidneys cannot hold onto water.
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The Shift: If a polyuric cat suddenly stops urinating as frequently, owners often mistake this for improvement. They think, "Oh good, he isn't flooding the litter box anymore."
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The Reality: This is actually a sign of terminal collapse or severe Acute Kidney Injury (AKI) on top of the chronic disease. The filtration rate has dropped so low that the kidneys can no longer physically push fluid through the tubes.
The "Fluid Trap" Oliguria changes the rules of treatment completely.
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Polyuric Cats: These cats need fluids. You can give them SubQ fluids, and they will pee out the excess. It is hard to "drown" a polyuric cat.
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Oliguric Cats: These cats cannot excrete water. If you give the standard amount of fluids to an oliguric cat, the fluid stays trapped in their body. This rapidly leads to Volume Overload, causing fluid to back up into the lungs (Pulmonary Edema) and heart failure.
The Potassium Spike Because urine is the primary exit route for potassium, oliguria leads to rapid Hyperkalemia. Without urine flow to wash it out, potassium levels in the blood rise to lethal levels, which can stop the heart.