Kidney Function Tests: What Creatinine and eGFR Actually Tell You
Kidney disease is unusually quiet. Most people lose a substantial proportion of kidney function before developing any symptom at all, which means the test results are the symptom.
What kidneys do
Beyond filtering waste, kidneys regulate fluid balance, control blood pressure, manage electrolytes such as sodium and potassium, activate vitamin D, and signal the bone marrow to produce red blood cells. This is why kidney failure produces anaemia and bone disease, which surprises people who think of kidneys purely as filters.
Creatinine
Creatinine is a waste product of muscle metabolism, produced at a fairly steady rate and cleared by the kidneys. If kidneys filter less, creatinine rises.
The problem is that creatinine depends heavily on muscle mass. A muscular young man and a frail elderly woman with identical kidney function will have very different creatinine levels. A thin elderly person can have significantly reduced kidney function with a creatinine that sits inside the normal range.
Creatinine also rises only after a substantial loss of function. It is an insensitive early marker.
eGFR
Estimated glomerular filtration rate converts creatinine into an estimate of filtering capacity, adjusting for age and sex. It is reported in mL/min/1.73m² and is far more useful than creatinine alone.
Stages of chronic kidney disease by eGFR:
- G1: 90 or above, normal filtration. CKD only if other evidence of damage such as protein in urine
- G2: 60 to 89, mildly reduced. Again, only CKD with other evidence of damage
- G3a: 45 to 59, mild to moderate reduction
- G3b: 30 to 44, moderate to severe reduction
- G4: 15 to 29, severely reduced
- G5: below 15, kidney failure
Note that stages G1 and G2 require additional evidence of kidney damage before being called disease. An eGFR of 75 in a healthy 70-year-old with no protein in the urine is usually age-appropriate, not a diagnosis.
Older race-based adjustments to eGFR have been removed from most modern equations.
The test most people are not offered
Urine albumin-to-creatinine ratio, or uACR.
Albumin is a protein that should not pass into urine. When the filtering units are damaged, it leaks. This leak appears before eGFR falls, which makes uACR an earlier marker than creatinine-based estimates. It is also a strong independent predictor of cardiovascular risk.
Categories:
- A1: below 30 mg/g, normal to mildly increased
- A2: 30 to 300 mg/g, moderately increased
- A3: above 300 mg/g, severely increased
A single abnormal result should be repeated, since exercise, fever, infection and menstruation can cause transient elevation.
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Chronic kidney disease is staged by both numbers together, written as something like G2 A2. Two people with the same eGFR can have very different prognoses depending on albuminuria. If you have diabetes or hypertension and have never had a uACR, ask for one.
A urine dipstick is not enough
Dipstick protein testing misses moderately increased albuminuria entirely. It detects larger amounts. If early kidney damage is the question, the dipstick answers it poorly.
Who should be tested
- Everyone with diabetes, at least annually
- Everyone with hypertension
- Cardiovascular disease or heart failure
- A family history of kidney disease
- Obesity
- Long-term use of NSAIDs or other nephrotoxic drugs
- Recurrent kidney stones or urinary infections
- Age over 60
- Autoimmune conditions such as lupus
What acute change means
A sudden rise in creatinine is acute kidney injury rather than chronic disease, and has different causes: dehydration, severe infection, obstruction from stones or an enlarged prostate, and medicines including NSAIDs, certain antibiotics, ACE inhibitors and contrast dye in combination with dehydration.
Acute kidney injury is often reversible if the cause is addressed quickly, which is why a new abnormal result is checked and acted on rather than simply repeated in three months.
Protecting kidney function
Blood pressure control is the single most important measure. Targets are usually tighter when albuminuria is present.
Blood sugar control in diabetes.
Specific medicines. ACE inhibitors or ARBs reduce protein leak and slow progression and are first choice when albuminuria is present. SGLT2 inhibitors, originally diabetes drugs, have been shown to slow kidney disease progression substantially, including in people without diabetes, and are now a mainstay.
Avoid regular NSAIDs. Ibuprofen, diclofenac and similar drugs taken frequently are a common and avoidable contributor to kidney damage, particularly in people already at risk.
Hydration, sensibly. More water does not improve kidney function in a healthy person, but dehydration is harmful, particularly in heat or illness.
Salt reduction, which supports blood pressure control.
Protein, in moderation. Very high protein intake increases filtration workload. Severe restriction is only advised under supervision in advanced disease.
Stop smoking, which accelerates progression.
Check supplements and herbal products. Several, including some traditional preparations, are nephrotoxic.
Sick day rules
During significant vomiting, diarrhoea, fever or dehydration, some medicines should be temporarily paused because the combination with dehydration can injure kidneys. These commonly include ACE inhibitors, ARBs, diuretics, metformin, SGLT2 inhibitors and NSAIDs. Ask your doctor which of your medicines this applies to before you need to know.
When to seek attention
- Markedly reduced urine output
- Blood in urine
- Visible foaming of urine, which can indicate protein
- Swelling of ankles, face or around the eyes
- Unexplained breathlessness
- Persistent nausea with fatigue and itching
The short version
eGFR is more useful than creatinine, and urine albumin is more sensitive than both. If you have diabetes or high blood pressure and have never had a urine albumin-to-creatinine ratio, that is the test to ask for. Kidney disease is almost entirely silent in its treatable phase, which is the whole reason for testing people who feel fine.