GFR (eGFR) test explained (GFG)

The glomerular filtration rate (GFR, usually reported as eGFR — estimated glomerular filtration rate) shows how much blood passes each minute through the filtering units of the kidneys, the glomeruli. It is the main indicator used to assess kidney function. GFR is not measured directly in everyday practice — it is calculated (which is why it is called estimated) from the amount of creatinine in the blood together with age and sex, using a formula. Creatinine is a by-product of muscle metabolism that the kidneys constantly remove from the blood — when kidney function declines, creatinine builds up in the blood and the calculated GFR falls. Sometimes a creatinine clearance test, which compares the amount of creatinine in urine and in blood, is done for a more precise assessment. GFR is used not only to establish whether kidney disease is present, but also to grade its severity (from G1 to G5) and to choose how often treatment or monitoring is needed.

• When kidney function needs to be assessed before prescribing medicines that can affect the kidneys or are cleared through them
• In people with diabetes or raised blood pressure — as periodic monitoring of the state of the kidneys
• In cardiovascular disease, or when there has been kidney disease in the family
• When general blood or urine tests show deviations that could point to a kidney problem
• In people taking medicines long term that can harm the kidneys (for example some non-steroidal painkillers or lithium preparations)
• When monitoring already known chronic kidney disease — how often testing is needed is decided by the doctor

A reduced GFR indicates a weakened filtering function of the kidneys. When it stays below 60 ml/min/1.73 m² for three months or longer, it is considered chronic kidney disease under the NHS classification running from G1 to G5. Possible causes include raised blood pressure, diabetes and raised cholesterol, which damage the blood vessels and filters of the kidneys.

Possible causes

• Raised blood pressure — it damages the blood vessels of the kidneys
• Diabetes — excess glucose damages the filters of the kidneys
• Raised cholesterol — fatty deposits build up in the blood vessels
• Kidney infections and glomerulonephritis (inflammation of the kidneys)
• Inherited polycystic kidney disease
• Obstruction to the flow of urine — kidney stones, an enlarged prostate
• Long-term use of some medicines (for example lithium preparations or non-steroidal painkillers)

Possible symptoms

• At an early stage there are usually no symptoms
• At an advanced stage — weight loss and loss of appetite
• Swelling in the ankles, feet or hands from fluid retention
• Shortness of breath
• Tiredness
• Blood in the urine, passing urine more often, especially at night
• Sleep problems, itchy skin, muscle cramps, nausea, headaches

• Whether the reduction persists on a repeat test after 3 months — a single result does not establish a stage
• Blood pressure, glucose and cholesterol
• The amount of protein in the urine (the albumin-to-creatinine ratio) — an additional marker of kidney damage
• Medicines being taken that can affect the kidneys
• How often to repeat the test is decided by the doctor according to the stage

GFR bands and what they mean

GFR above 90 ml/min/1.73 m² (G1 — normal or increased function): Under the KDIGO and NHS classification, a GFR above 90 ml/min/1.73 m² corresponds to stage G1 — normal or increased filtering function of the kidneys. The value itself does not indicate chronic kidney disease: stage G1 becomes part of a disease only when other markers of kidney damage, for example a raised amount of protein in the urine, are found alongside it for at least three months. In the KDIGO staging system for chronic kidney disease, G1 marks a normal or even increased glomerular filtration rate, and unlike the other stages it is not in itself a sign of a disorder — it is simply the upper limit of healthy filtration. For G1 to be classed as chronic kidney disease, there has to be other structural or functional evidence of damage as well, most often a raised urine albumin-to-creatinine ratio or changes on kidney imaging, persisting for three months or longer. If a GFR above 90 is found with no other deviations — no protein in the urine, no blood pressure or glucose problems — the result itself usually calls for no extra monitoring. The doctor weighs the whole context: eGFR is calculated from creatinine, which depends on muscle mass, age and sex, so the result of a very muscular or young person can differ from the average with no pathology at all. One high GFR result does not replace a thorough assessment of kidney function — it is always read together with urine tests and blood pressure and, when there are other health circumstances that can affect the kidneys, repeated over time.

GFR 60–89 ml/min/1.73 m² (G2 — mildly reduced): A GFR of 60–89 ml/min/1.73 m² corresponds to KDIGO and NHS stage G2 — a mild reduction in the filtration rate. According to the NHS, this stage is considered chronic kidney disease only when other signs of kidney damage are present as well, for example protein in the urine, and the reduction persists for three months or longer. Without such signs it can be an age-related, insignificant change. Stage G2 covers a wide range — from almost normal to slightly reduced function. The NHS stresses that this result alone does not confirm chronic kidney disease: either additional evidence of kidney damage is needed (protein in the urine, structural changes), or the reduction itself has to persist for at least three months, so that a temporary cause such as dehydration or a recent infection can be ruled out. GFR naturally declines gradually with age even without any kidney disease, so in older people a value in this range is often assessed together with other indicators rather than in isolation. The doctor usually compares the new result with earlier tests, if there are any, to see whether the change is sudden or gradual and stable. Because this stage often has no symptoms at all, it is most often picked up incidentally on blood tests done for other reasons — one of the reasons periodic testing is recommended for people with diabetes or raised blood pressure.

GFR 45–59 ml/min/1.73 m² (G3a — moderately reduced): A GFR of 45–59 ml/min/1.73 m² corresponds to KDIGO and NHS stage G3a — a moderate reduction in the filtration rate. Unlike G1–G2, this stage on its own, once it has persisted for three months or longer, is already considered chronic kidney disease, even if there are no other signs of damage. Possible causes include raised blood pressure and diabetes. G3a is the threshold from which the NHS classification recognises chronic kidney disease on the GFR value alone, without requiring additional evidence of damage — provided the reduction is confirmed by a repeat test after three months. It means the kidneys are already filtering noticeably less than usual, but there are usually no symptoms yet, or they are very faint. At this stage it is especially important to assess what caused the reduction — this can be long-term uncontrolled blood pressure or glucose damaging the blood vessels and filtering units of the kidneys. Protein (albumin) in the urine is usually assessed alongside GFR: it helps to judge how quickly the stage may progress and how often monitoring will be needed. A single result in this range does not yet say whether the change is stable or progressing — that becomes clear only by comparing several tests over time. Creatinine is often reassessed alongside GFR as well, to make it clearer whether a temporary cause or a real fall in filtration is behind the change.

GFR 30–44 ml/min/1.73 m² (G3b — moderately to severely reduced): A GFR of 30–44 ml/min/1.73 m² corresponds to KDIGO and NHS stage G3b — a moderate to severe reduction in the filtration rate. This is an advanced stage of chronic kidney disease, in which kidney function is reduced by more than half compared with normal, so more frequent monitoring and a broader assessment of related indicators are usually needed. G3b is separated from G3a precisely because the likelihood of complications — such as anaemia, changes in bone metabolism or an electrolyte imbalance — rises noticeably at this stage, so at this point doctors usually widen the range of tests beyond GFR and creatinine alone. First symptoms also appear more often at this stage — tiredness, swelling in the ankles or feet, changes in appetite — but they are not universal and can go unnoticed for a long time. Because the speed of progression differs greatly between people, what matters most is following the direction and pace of change across several measurements rather than a single result. In assessing stage G3b, the amount of protein in the urine, blood pressure and co-existing conditions are considered together, because all of them together determine how quickly the stage may change and what monitoring interval is most suitable. It may also be discussed whether other preparations taken at the same time contributed to the reduction; their effect on the kidneys is worth discussing with a doctor.

GFR 15–29 ml/min/1.73 m² (G4 — severely reduced): A GFR of 15–29 ml/min/1.73 m² corresponds to KDIGO and NHS stage G4 — a severe reduction in the filtration rate. This is an advanced stage of chronic kidney disease, in which kidney function is very markedly reduced, so close specialist care and preparation for a possible further decline in function are usually needed. At stage G4, kidney function is reduced to the point where the body often can no longer fully maintain the balance of fluids, electrolytes and metabolic waste products on its own. Symptoms such as shortness of breath, tiredness, swelling or nausea appear more often than at earlier stages, but their intensity can vary a great deal. At this stage care is usually handed over to, or closely coordinated with, a kidney specialist (a nephrologist), who assesses the speed of progression and plans the next steps, including possible preparation should function decline further. Other indicators are assessed as well — anaemia, bone metabolism, potassium and other electrolyte levels. Because G4 is the last stage before the most severe kidney failure (G5), monitoring at this stage is usually more frequent than at earlier ones, so that changes are noticed in time. It is also important to discuss which tests are needed for further monitoring and how often, since the situation at this stage can change relatively quickly.

GFR below 15 ml/min/1.73 m² (G5 — kidney failure): A GFR below 15 ml/min/1.73 m² corresponds to KDIGO and NHS stage G5 — kidney failure. According to the NHS, at this stage the kidneys have lost almost all of their filtering function. This is the most severe stage of chronic kidney disease, in which the body is usually no longer able to maintain the balance of fluids and metabolic waste products on its own. Stage G5, by the NHS definition, means that the kidneys have lost almost all of their filtering ability. At this point the body can no longer effectively remove waste products itself or maintain the balance of fluids and electrolytes, so close specialised care is usually needed. The next step is urgent coordination of care with a kidney specialist (a nephrologist). This stage is most often established in someone who already has a longer history of chronic kidney disease and in whom a gradual decline in GFR has been followed through the earlier stages, although in some cases function can decline faster. Decisions about further care at this stage are made individually, taking into account overall health, symptoms and other tests. A single blood test result in this range is always assessed together with the clinical picture — symptoms, other laboratory indicators and how kidney function has developed previously. Informing family members or close relatives about the situation also often becomes an important part of preparing together for further care.

What is usually assessed at stage G4

• Coordination of care with a kidney specialist (a nephrologist)
• Electrolytes (especially potassium), anaemia and bone metabolism indicators
• Fluid balance and blood pressure control
• A comparison with earlier GFR results — assessing the speed of progression
• Planning for a possible further decline in function together with a specialist