A low GFR means your kidneys aren’t filtering blood as efficiently as they should be. GFR stands for glomerular filtration rate, and it measures how many milliliters of blood your kidneys clean per minute. A normal GFR is above 90, and anything below 60 that persists for three months or more is classified as chronic kidney disease. A single low reading, though, doesn’t always mean your kidneys are in trouble.
What Your GFR Number Actually Tells You
Your GFR result is reported as an “estimated” GFR (eGFR) because it’s calculated from a blood test, not measured directly. The blood test checks creatinine, a waste product your muscles produce at a fairly steady rate. When kidneys work well, they filter creatinine out quickly. When they don’t, creatinine builds up, and your eGFR number drops.
Kidney function is divided into five stages based on your eGFR:
- Stage 1 (eGFR 90 or above): Mild kidney damage is present, but your kidneys still filter well. No symptoms.
- Stage 2 (eGFR 60 to 89): A modest decline in function. Still typically no symptoms.
- Stage 3 (eGFR 30 to 59): Moderate loss of function. You may start feeling tired, weak, or notice swelling in your hands or feet.
- Stage 4 (eGFR 15 to 29): Severe loss of function. Swelling, back pain, and fatigue are common.
- Stage 5 (eGFR below 15): Kidney failure. Waste products build up in the blood and can make you very sick.
Most people feel nothing in stages 1 and 2, which is why a low GFR often shows up as a surprise on routine bloodwork. Symptoms tend to appear around stage 3, when kidneys have lost roughly half their filtering capacity.
A Low GFR Doesn’t Always Mean Kidney Disease
The standard eGFR formula uses creatinine, and creatinine levels are affected by more than just kidney function. Muscle mass is a big one. People with above-average muscle produce more creatinine, which can push their eGFR artificially lower. Research using body composition scans found that eGFR dropped by roughly 6 points for every additional 10 kg of lean muscle mass, independent of actual kidney function. In highly muscular people, the test’s ability to correctly rule out kidney disease fell to just 47%.
On the flip side, someone with very low muscle mass (from aging, chronic illness, or malnutrition) can get a falsely reassuring eGFR that overestimates their real kidney function. A high-protein meal shortly before the blood draw can also skew results. If your doctor suspects your creatinine-based eGFR is misleading, they can order a second marker called cystatin C. Combining creatinine and cystatin C produces a more accurate estimate, and the National Institute of Diabetes and Digestive and Kidney Diseases recommends this combined approach when results are close to a treatment threshold.
Common Causes of Kidney Function Decline
The two leading causes of chronic kidney disease are high blood pressure and diabetes. Over years, uncontrolled blood sugar damages the tiny blood vessels inside the kidneys, and sustained high blood pressure puts mechanical stress on them. Together, these two conditions account for the majority of cases.
Other causes include inherited conditions like polycystic kidney disease, autoimmune diseases like lupus, recurring kidney infections, and urinary tract obstructions such as kidney stones or an enlarged prostate. Certain medications can also lower GFR. Common over-the-counter pain relievers like ibuprofen and naproxen (NSAIDs) reduce blood flow within the kidneys and can cause damage with prolonged use. Some antibiotics, antivirals like acyclovir, and the mood stabilizer lithium are also known to stress the kidneys, especially at high doses or over long treatment courses. The general clinical threshold is that medications requiring kidney clearance need dose adjustments once eGFR falls below 60.
Sudden vs. Gradual Drops in GFR
A low GFR can reflect two very different situations. A sudden drop, called acute kidney injury, can happen over hours or days from dehydration, a severe infection, a medication reaction, or a blockage in the urinary tract. With prompt treatment, kidney function often returns to normal because the damage is potentially reversible.
A gradual decline over months or years points toward chronic kidney disease, where ongoing damage slowly replaces healthy kidney tissue with scar tissue. This scarring (fibrosis) is accompanied by loss of tiny blood vessels inside the kidney, creating zones of low oxygen that drive further damage. Once established, this process is not fully reversible.
The two conditions also feed into each other. People who already have chronic kidney disease are less able to recover fully from an acute injury. Instead of bouncing back, the kidney tends to scar further, accelerating the long-term decline. Complete recovery from acute kidney injury is less common than doctors previously assumed, and even a single episode can leave lasting reductions in function.
What Happens After a Low GFR Result
A single low reading usually triggers a repeat test in a few weeks or months. GFR that stays below 60 on two or more tests at least 90 days apart meets the criteria for chronic kidney disease. Your doctor will also check for protein in your urine using a urine albumin-to-creatinine ratio (UACR). This test matters because albumin leaking into urine is a separate marker of kidney damage and a strong predictor of how quickly things may progress. In a large study of people with chronic kidney disease, higher albumin levels at diagnosis were associated with a greater risk of losing half their remaining kidney function, needing dialysis, or dying. Reducing albumin levels through treatment correlated with better outcomes for both kidney function and heart health.
Together, eGFR and UACR paint a much fuller picture than either one alone. You can have a normal eGFR but significant albumin in your urine (early damage), or a low eGFR with no albumin (which may reflect normal aging or muscle mass effects rather than progressive disease).
Protecting Kidney Function With a Low GFR
Managing blood pressure and blood sugar, if either is elevated, is the single most impactful thing you can do to slow further decline. For most people with kidney disease, these two factors drive the majority of ongoing damage.
Diet adjustments become increasingly important as GFR drops. The key nutrients to watch are sodium, protein, potassium, and phosphorus. Excess sodium raises blood pressure and increases fluid retention, both of which strain the kidneys. Protein intake often needs to be moderated because damaged kidneys struggle to clear the waste products of protein metabolism. Potassium and phosphorus, normally filtered out easily, can accumulate to dangerous levels when kidney function is significantly reduced, affecting heart rhythm and bone health respectively. The specific limits vary by stage and by individual, so working with a dietitian who specializes in kidney disease is more useful than following generic guidelines.
Avoiding NSAIDs (ibuprofen, naproxen) is a straightforward protective step. These drugs reduce blood flow to the kidneys and can accelerate damage, especially when GFR is already below 60. If you take any regular medications, including supplements, your doctor or pharmacist can flag ones that need dose adjustments based on your current GFR.
Staying hydrated, maintaining a healthy weight, not smoking, and getting regular physical activity all support kidney health, though none of these will reverse established damage. The goal with chronic kidney disease is to slow progression, and many people live for decades at stage 2 or 3 without ever reaching kidney failure.