The rest of this guide walks through what each part of the test measures, how the sample is taken, what can move a result without the kidneys changing, what the categories mean in plain words, and the point at which a result is worth taking to a clinician.
A kidney function blood test checks how efficiently the kidneys are clearing waste out of the blood, by measuring a small number of substances that accumulate when filtration slows. The headline figure on most reports is eGFR, short for estimated glomerular filtration rate, and the important detail is that it is calculated rather than directly measured.
Each kidney contains an enormous number of microscopic filtering units called nephrons. Blood arrives at a tuft of tiny capillaries known as the glomerulus, water and small molecules are pushed through into a tubule, and the tubule then reclaims most of the water, salt and glucose the body still needs. What the body does not need, including creatinine and urea, stays in the fluid and leaves as urine. When filtration slows, those waste molecules rise in the blood, and that rise is what the test detects.
So a standard kidney panel is doing three jobs at once. It measures a filtration marker, almost always creatinine and sometimes cystatin C. It converts that marker into an estimated filtration rate using an equation built around age and sex. And it usually reports urea and electrolytes alongside, which say something about fluid balance and how the kidneys are handling salt, acid and potassium.
A urine test sits beside the blood work rather than inside it. Filtration speed and filter leakiness are two different things, and a kidney can be leaking albumin while still clearing waste at a normal rate.
| Test | Sample | What it reflects | Main limitation |
|---|---|---|---|
| Serum creatinine | Blood | Waste from muscle turnover that the kidneys clear | Production depends on muscle mass, diet and recent exercise |
| eGFR | Calculated from creatinine, cystatin C, or both | An estimate of filtration rate scaled to a standard body size | It is an estimate, and least precise at the high end of the range |
| Cystatin C | Blood | Filtration, using a protein made by nearly all cells | Run less often, and influenced by thyroid status, inflammation and body fat |
| Urea, often reported as BUN | Blood | Waste from protein handling, cleared by the kidneys | Moves with hydration and protein intake |
| Urine albumin to creatinine ratio | Urine | Albumin leaking through the filtration barrier | Needs repeating, since fever, infection and hard exercise can raise it briefly |
GFR is estimated because measuring it directly is slow, expensive and impractical for routine care. A true measurement requires giving a marker substance that the kidneys clear in a predictable way, then collecting timed blood or urine samples over several hours to see how fast it disappears.
The workaround is elegant. The body produces creatinine at a reasonably steady rate, and the kidneys clear it. If production is steady and the blood level climbs, clearance must have fallen. Feed the creatinine value into an equation that adjusts for age and sex, and you get an estimate of filtration rate that tracks the real thing closely enough for most decisions.
That is the whole logic behind the number on your report, and it explains most of its weaknesses. The estimate is only as good as the assumption that your creatinine production is typical for your age and sex. If it is not, the estimate drifts. The difference between the measured quantity and the calculated one is worth understanding properly, which is covered in more depth in is eGFR the same as GFR.
One more quirk. eGFR is reported scaled to a standard body surface area, usually written as mL/min/1.73 m². That standardisation lets results be compared between a small adult and a large one, but it also means the number is not a literal measurement of your own kidneys in millilitres per minute.
Serum creatinine shows the balance between how much creatinine your muscles are producing and how much your kidneys are clearing, which is why it is informative and frustrating in equal measure. Creatinine is the breakdown product of creatine phosphate, the rapid energy store inside muscle. Muscle turns over a small, fairly constant fraction of that store every day, the waste enters the bloodstream, and the glomerulus filters it out with a small additional amount secreted by the tubules.
Because production scales with muscle mass, two people with identical kidney function can sit at genuinely different creatinine values. A heavily muscled adult runs higher. A frail adult with little muscle runs lower. Neither value says anything about filtration until it is read in context.
The relationship between creatinine and filtration is also not a straight line. In the early part of the range, filtration can fall meaningfully before creatinine rises much at all, because the remaining nephrons compensate. Later in the range, small further losses of filtration produce large jumps in creatinine. This is why a creatinine value that has moved from the low end of normal to the high end of normal can matter more than it looks, and why clinicians watch the trend across repeat tests rather than a single reading.
The practical upshot is simple. Creatinine on its own is a raw input. The eGFR calculated from it is the interpreted output, and even that interpretation carries assumptions worth knowing about.
The lab runs your creatinine value through a validated estimating equation that also uses your age and sex, then reports the result scaled to standard body size. You do not need to do any arithmetic, and in most systems the eGFR appears automatically whenever a creatinine is ordered.
The equations are built by comparing blood marker levels against directly measured filtration rates in large groups of people, then fitting a formula that predicts the measured value as closely as possible. Age enters the equation because creatinine production tends to decline as muscle mass declines with age. Sex enters it for the same reason, since average muscle mass differs. Current clinical guidance supports estimating equations that do not apply a race coefficient 1.
There are separate equations for creatinine alone, for cystatin C alone, and for the two markers combined. The combined version is generally the most reliable, because the two markers have different sources of error and averaging them cancels some of the noise.
What an equation cannot do is know your individual circumstances. It does not know your body composition, your diet, whether you lifted heavy yesterday, or whether you are mid way through an acute illness. It assumes you are in a steady state, meaning your creatinine level has had time to settle. During a rapid change in kidney function, the blood level lags behind reality, so an eGFR calculated in the middle of an acute event can read better than the kidneys are actually doing at that moment.
The eGFR categories are a shorthand for how much filtration capacity is estimated to remain, running from normal through mildly reduced, moderately reduced, severely reduced and kidney failure. They are descriptive labels, not diagnoses, and they only carry weight once a result has been confirmed as persistent.
In everyday language, the top category means filtration is in the normal or high range. The next means filtration is slightly below what the equation expects, which on its own is common and often unremarkable. The middle categories mean filtration is meaningfully reduced and the kidneys have less reserve than they once did. The lowest categories mean filtration is severely reduced, and that is a situation for specialist input.
Two features of the system matter more than the labels. First, time. Chronic kidney disease is defined by abnormalities of kidney structure or function that persist for more than three months, which is why a single reduced value is a prompt to recheck rather than a conclusion 1. The original classification of kidney disease into stages based on filtration level and persistence set that framework decades ago 6.
Second, filtration is only half the grid. Modern staging combines the filtration category with an albuminuria category, because the two carry different information and together describe risk better than either alone 3. Someone with mildly reduced filtration and no albumin leak sits in a very different position from someone with the same filtration and heavy albumin leak.
Typical lab reference ranges for eGFR run from roughly 60 to 130 mL/min, and many laboratories simply report anything above 60 as "greater than 60" rather than giving a precise figure, because the estimate loses precision at the upper end. FitScript's optimal range for eGFR is 90 to 120 mL/min. Optimal ranges are optimization targets, not diagnostic cut-offs, and standard reference ranges vary by laboratory, age and sex.
The gap between those two framings is worth sitting with. A standard reference range answers the question of whether a value looks like most people's. An optimization target answers a narrower question about where you would prefer to sit if you are tracking a marker over years. Neither replaces clinical judgement about an individual result.
It also helps to know that filtration gradually declines with age in most people, so an eGFR that would be unusual in a twenty year old can be entirely expected later in life. That is why age is inside the equation and why clinicians read a value against a person's own history rather than against a single universal cut-off. A stable value that has sat in the same place for five years tells a different story from the same value reached after a steady slide.
If your report shows only "greater than 60", that is not the lab being unhelpful. It reflects a real limit of the method.
Several everyday things shift creatinine, and therefore eGFR, with no change in kidney function whatsoever. Knowing them prevents a lot of unnecessary alarm and a lot of false reassurance.
Muscle mass is the biggest. More muscle means more creatinine production, a higher blood level and a lower calculated eGFR. Research suggests that cystatin C is less influenced by muscle mass than creatinine, which is precisely why it exists as an alternative marker 5. The mirror image also applies, so very low muscle mass can produce a flatteringly high estimate.
Recent hard exercise raises creatinine for a period afterwards, particularly resistance work or long endurance efforts, because muscle turnover increases. A large meat meal in the day before a draw can do the same, since cooked meat contains creatinine that is absorbed directly. Creatine supplementation raises the substrate pool and can nudge the value up as well.
Hydration matters in both directions. Being markedly dehydrated concentrates the blood and reduces the volume reaching the kidneys, which raises creatinine temporarily. Being very over hydrated can dilute it slightly.
Pregnancy lowers creatinine because filtration rises. Acute illness, fever and vomiting can raise it transiently. None of these are reasons to dismiss a result, and none of them are reasons to game a test. They are reasons to standardise the conditions, keep hydration normal, avoid an unusually hard training session or a large meat meal the day before, and repeat the test the same way next time.
Cystatin C is used when a creatinine based estimate is likely to be misleading, or when an accurate filtration estimate will change a decision. Clinical guidance supports confirmatory testing with cystatin C in adults where a creatinine based eGFR alone is not precise enough for the decision at hand 1.
Cystatin C is a small protein produced at a fairly constant rate by nearly all nucleated cells in the body, freely filtered at the glomerulus and then broken down in the tubules. Because its production is not tied to muscle, it sidesteps the biggest weakness of creatinine. That makes it useful in people with unusually high or unusually low muscle mass, in older adults with low muscle, in people with limb loss, and in anyone whose creatinine based number does not match the clinical picture.
It is not a perfect marker either. Studies indicate that cystatin C levels can be influenced by factors including thyroid function, inflammation and body fat 5. That is why the combined equation using both markers is generally regarded as the most reliable routine estimate, since the two error sources do not overlap.
In practice, a reasonable sequence looks like this. Creatinine and its eGFR are the first look. If the result is borderline, if it conflicts with the rest of the picture, or if the person sits at an extreme of body composition, cystatin C is added and a combined estimate is produced. A clinician decides whether that second test is warranted.
A urine albumin test belongs alongside the blood work because filtration rate and filter integrity are separate properties, and a kidney can leak protein while still clearing waste normally. Measuring only eGFR misses that entirely.
Albumin is a medium sized blood protein that a healthy glomerular barrier holds back. When the barrier is damaged, small amounts begin to appear in urine, often long before filtration falls. The usual test is the albumin to creatinine ratio on a spot urine sample, which corrects for how dilute the sample is so that a mid morning sample can be compared with an early one.
This matters because of what albuminuria predicts. Lower eGFR and higher albuminuria are each associated with higher risk of all-cause mortality, cardiovascular mortality and kidney failure, and the two markers add information independently of one another 2. In pooled general population data, lower eGFR and higher albuminuria were associated with increased all-cause and cardiovascular mortality 4. Research suggests that combining eGFR with albuminuria improves prediction of cardiovascular outcomes compared with using either measure alone 7.
A single raised albumin ratio does not settle anything. Hard exercise, fever, infection, menstruation and very high blood sugar can all push it up temporarily, so the test is repeated to establish whether the leak is persistent. Two out of three raised samples over a few months is the usual standard of persistence.
The blood sample is an ordinary venous draw from the arm, takes a couple of minutes, and goes into a standard chemistry tube alongside whatever else has been ordered. The urine component, when ordered, is a small spot sample collected into a sterile container.
Fasting is not required for creatinine or cystatin C specifically. Many people are asked to fast because kidney markers are bundled into a wider panel that includes glucose and lipids, so follow whatever instruction the laboratory gives. What is worth standardising is the controllable stuff described earlier, namely hydration, recent training load and a very high protein meal the evening before.
Repetition is not bureaucracy. It is built into how these results are interpreted, for three reasons. The first is biological variability, since creatinine fluctuates day to day in the same person even with completely stable kidneys. The second is analytical variability between laboratories and methods, which is why comparing results across providers is less reliable than comparing within one. The third is the definition itself, since persistence over more than three months is what separates a chronic finding from a transient one 6.
There is also a sequencing point. An eGFR drawn during a vomiting illness, a dehydrating event or a hospital admission reflects that moment rather than your baseline. A clinician usually wants a repeat once the acute situation has resolved before drawing conclusions about long term function.
A lower than expected eGFR reflects either genuinely reduced filtration or a creatinine level that is high for reasons unrelated to the kidneys, and separating the two is the entire job of interpretation.
On the genuine side, filtration can fall acutely, usually because less blood is reaching the kidneys, because something is obstructing outflow, or because the kidney tissue itself is under stress. It can also fall chronically and gradually, which is the pattern that staging systems were built to describe. On the artefact side, high muscle mass, a hard training block, a heavy meat intake or dehydration can all pull the estimate down without any change in the kidney.
The practical read is a three step one. What the value shows is a reduced estimate of filtration at this moment. What it may reflect ranges from normal variation and body composition through to reduced kidney reserve, and a single value cannot distinguish between those. What is worth doing next is a conversation with a clinician about repeating the test under standard conditions, adding a urine albumin ratio, and considering a cystatin C based estimate.
A higher than expected eGFR is less commonly discussed but worth understanding. It can reflect low muscle mass, which lowers creatinine production and inflates the calculated figure. It can also reflect the limits of the equations themselves, which were built mainly around people with reduced filtration and lose precision in the upper range. Increased filtration pressure early in some metabolic conditions can also raise the figure. A very high number is therefore not automatically a gold star, and it is worth asking a clinician what it means in your context.
Kidney markers make far more sense as a cluster than as single numbers, because the kidney does several jobs and failure in one shows up before failure in another. A clinician decides which of these are relevant for a given person.
Urea, often reported as BUN, is a second waste marker that moves with hydration and protein intake as well as filtration, and the ratio between urea and creatinine can hint at whether a change is about blood flow or about the kidney itself. Sodium and potassium reflect how the kidney is handling salt and are read carefully whenever filtration is reduced. Bicarbonate reflects acid handling. Calcium, phosphate and vitamin D sit downstream of kidney function because the kidney activates vitamin D and clears phosphate. Parathyroid hormone often shifts before calcium and phosphate do.
Serum albumin says something about protein status and, alongside urine albumin, about protein loss. Uric acid is cleared by the kidney and tends to rise as filtration falls. Haemoglobin matters because the kidney produces the hormone that drives red cell production, so anaemia can appear as function declines.
Blood testing cannot tell you why kidney function is where it is, and that is its most important limit. It quantifies output, not cause. Structural questions, such as stones, cysts, obstruction or size differences between the two kidneys, need imaging. Tissue level questions sometimes need a biopsy. Both of those are clinician decisions.
It also cannot separate acute from chronic on a single sample. The same eGFR could represent a stable, long standing baseline or a sharp recent drop, and only a previous result or a repeat can tell you which.
It cannot confirm a normal filter from filtration alone, because albumin can leak while eGFR sits comfortably in the normal range, which is exactly why the urine test exists and why guidance pairs the two measures in staging 3.
And it cannot escape being an estimate. The equations perform well across populations, less well at the extremes of body composition, and less precisely in the upper part of the range. A value of 95 and a value of 103 on consecutive tests are not meaningfully different, and chasing small fluctuations is a reliable way to generate anxiety without generating insight. What is informative is the direction of travel across several results taken under similar conditions, read alongside urine albumin and the rest of the metabolic picture.
Any eGFR below the standard reference range, any detectable albumin in urine, or a clear downward trend across repeat tests is worth taking to a clinician, even when you feel entirely well. Reduced filtration frequently produces no symptoms at all until it is well advanced, which is the whole argument for testing rather than waiting.
Some situations deserve prompt attention rather than a routine appointment. A sudden large fall in eGFR compared with a recent result, a marked drop in how much urine you are passing, new swelling around the ankles, eyes or face, persistently foaming urine, blood in the urine, or feeling significantly unwell alongside an abnormal result are all worth discussing with a clinician promptly. Never ignore an out of range result because you feel fine, and never assume an out of range result means disease, since both errors are common.
It is also worth a conversation if you sit at an extreme of body composition, because the creatinine based estimate is least reliable there and a cystatin C based or combined estimate may give a truer picture. Bring your previous results if you have them, since trend beats snapshot every time.
This page is educational and is not medical advice. Discuss your own results, and any decision about next steps, with a clinician or licensed provider who can see your full history. Any therapy that might be considered is a decision a licensed provider makes after evaluating whether it is appropriate for you.
For the marker pages behind this guide, see the biomarker entries for eGFR, creatinine and cystatin C, and the explainer on how the estimate differs from true filtration rate. FitScript tests 150+ biomarkers per year across its membership, including the markers grouped in its renal function panel.
Fasting is not required for creatinine or cystatin C themselves. You may still be asked to fast because kidney markers are often bundled into a wider panel that includes glucose and lipids, so follow the instruction the laboratory gives you. Keeping hydration normal and avoiding an unusually hard training session or a very large meat meal the day before helps make results comparable over time.
GFR is the actual rate at which your kidneys filter blood. Measuring it directly needs a marker substance and timed sampling over several hours, so it is rarely done outside specialist settings. eGFR is an estimate of that rate, calculated from a blood marker plus your age and sex, and scaled to a standard body size. For most purposes the estimate is close enough to guide decisions.
No. A single value is a snapshot and can be pulled down by dehydration, a recent heavy training session, a high meat intake or an acute illness. A clinician looks for a result that persists for more than three months, checks a urine albumin test, and compares the value with your previous results before drawing any conclusion. Repeating the test under standard conditions is the usual next step.
More muscle produces more creatinine, so the blood level rises and the calculated estimate falls, even though filtration has not changed. This is one of the clearest limits of a creatinine based estimate. A clinician may add a cystatin C based or combined estimate, since cystatin C production is not tied to muscle mass, to see whether the lower figure is real.
Many laboratories truncate reporting above 60 because estimating equations lose precision in the normal range. The lab is signalling that filtration appears to be in the normal range rather than pretending to a precision the method does not have. If you want a specific figure tracked over time, ask your clinician or lab whether a numeric result can be reported.
Often yes. Filtration speed and filter integrity are different properties, and albumin can leak into urine while the eGFR still sits in the normal range. The usual test is an albumin to creatinine ratio on a spot urine sample. A clinician decides whether to order it and whether to repeat it, since fever, infection and hard exercise can raise it temporarily.
Not necessarily. Low muscle mass lowers creatinine production and can inflate the calculated figure, and the equations are least precise at the top of the range. Increased filtration pressure in some metabolic situations can also raise it. A high number is worth reading in context with the rest of your results rather than taken as a gold star on its own.
A clinician or licensed provider does, after looking at your trend across repeat tests, your urine albumin result, your blood pressure, your medical history and the rest of your blood work. Testing describes what is happening. Deciding on any next step, including further testing, imaging, referral or any therapy, is a clinical decision made for you as an individual.