Kidney function tests get buried at the bottom of a routine metabolic panel, and most people never look at them until something is already wrong. They deserve more attention than that, because the early stages of kidney decline produce no symptoms at all.
What creatinine actually measures
Creatinine is a waste product from normal muscle activity, filtered out of your blood by the kidneys at a fairly constant rate. When kidney filtering slows down, creatinine backs up in the blood and the number rises. The catch is that creatinine also depends on how much muscle you have — a very muscular person and a frail elderly person can have meaningfully different kidney function at the exact same creatinine number, which is precisely why the raw number alone is a weak measure on its own.
Why eGFR is the number that actually matters
eGFR, estimated glomerular filtration rate, corrects for that by factoring in age, sex, and sometimes race or body size alongside creatinine, producing an estimate of how many millilitres of blood your kidneys filter per minute. Above 90 is generally normal. Between 60 and 89 with no other signs of kidney damage is usually not concerning on its own, since eGFR does decline gradually with normal ageing. Below 60, sustained over three months, meets the definition of chronic kidney disease. Below 30 is a serious stage that changes management significantly, and below 15 typically means preparing for dialysis or transplant.
The trend matters more than any single reading
A single eGFR of 65 in isolation tells me relatively little. The same number following a series of readings at 90, 82, 74, 65 tells a very different story than one following 68, 66, 65 — the first is a concerning downward trajectory, the second is stable. This is why I ask for repeat testing before reacting to one number, and why keeping old lab reports is genuinely useful.
What else I look at alongside these two numbers
Protein in the urine (albuminuria) is often the earliest sign of kidney stress, and it can appear before eGFR drops meaningfully. Blood pressure and kidney function are tightly linked in both directions — uncontrolled hypertension damages kidneys, and declining kidneys make blood pressure harder to control. Diabetes is the single most common cause of chronic kidney disease worldwide, which is why diabetic patients get kidney function checked at least annually regardless of symptoms.
What actually protects kidney function
Blood pressure control matters more than almost anything else for most people. Diabetic control matters equally for those with diabetes. Avoiding regular, heavy use of NSAIDs like ibuprofen is a genuinely underrated protective step — occasional use is fine for most people, but daily use over years is a preventable cause of kidney decline I see more often than I would like. Adequate hydration matters, though the popular advice to drink excessive amounts of water has no real basis; normal thirst-guided intake is sufficient for most healthy kidneys.
When to ask your doctor directly
If you have diabetes, high blood pressure, or a family history of kidney disease and have not had these numbers checked in over a year, ask for them explicitly at your next visit rather than assuming they were included.
This article is educational and does not replace an individual consultation. Kidney function interpretation depends on your full clinical picture, and only your doctor can tell you what your specific numbers mean for you.