Understanding how your kidneys are functioning often comes down to two key terms you’ll see on a lab report: Creatinine and GFR (Glomerular Filtration Rate).
Both are used to assess your kidney function, and thereby diagnose and monitor Chronic Kidney Disease (CKD).
Whilst they might seem like alphabet soup, they are the primary tools doctors (kidney and all doctors) use to determine how well your kidneys are working.
Here is a breakdown of what these markers are and why their relationship is so important in the assessment of kidney health.
Creatinine is a waste product that comes from the normal wear and tear on the muscles of the body. It is produced from creatine, a molecule used for muscle energy.
Because everyone has muscles, everyone has creatinine in their blood. Under normal circumstances, the kidneys filter creatinine from the blood and pass it out of the body through urine.
The Baseline: Since muscle mass stays relatively constant day-to-day, the production of creatinine is usually very steady.
The Red Flag: If your kidneys aren’t filtering effectively, creatinine begins to “pile up” in the bloodstream. Therefore, a high blood creatinine level is often the first physical sign that kidney function may be decreasing.
Normal blood creatinine is 60-120 mcmol/L.
What other factors affect the blood creatinine level?
But. Creatinine level does not just depend on someone’s kidney function. It is also affected by your:
If creatinine is a measurement of waste, GFR (Glomerular Filtration Rate) is a measurement of the speed at which the kidneys are working. It calculates how many milliliters of blood your kidneys filter per minute.
The “glomeruli” are the tiny loops of blood vessels that act as the mini-filters within the kidney. A GFR tells you how effectively these microscopic sieves are working.
Importantly, the GFR is calculated (mathematically by the computer) from the creatinine in an ‘inverse way’ (i.e. as creatinine goes up, GFR goes down); the higher the GFR, the lower the creatinine, the better – i.e. the better the kidneys are working.
What other factors affect the blood GFR level?
The most important thing to understand is that GFR is calculated using your creatinine level. Directly measuring GFR is actually quite difficult and invasive (it involves injecting special dyes).
Instead, scientists developed formulas—like the MDRD or CKD-EPI equations—to estimate GFR based on the amount of creatinine in your blood, while adjusting for age, biological sex, and sometimes body size.
The Inverse Relationship
As stated above, creatinine and GFR have an inverse relationship. As kidney function goes down, the GFR number drops, and the creatinine level rises.
Because of the ‘other factors’ that affect kidney function tests (like creatinine/GFR), doctors have to interpret the blood levels with the patient in front of them.
An Example
A creatinine level of 130 mcmol/L (above normal) might be perfectly healthy for a 25-year-old male athlete with high muscle mass, but that same 120 could signal significant chronic kidney disease (CKD) in an 80-year-old woman with very little muscle.
Because creatinine production depends on muscle mass (and other factors), the GFR calculation helps to “level the playing field” (but it is not perfect). Nonetheless, it allows doctors to see past the raw waste measurement and understand the actual performance of the organ.
Patients often find it very confusing to have two markers to assess kidney function – especially has they have different behaviours (creat goes up, and GFR goes down) as CKD worsens
But both measure the same thing.
So many doctors and nurses are now simplifying the information, and refer only to the GFR (or creatinine).
Some call the GFR the ‘percentage kidney function’ as ‘normal’ is about 100 ml/min. This is controversial for 2 reasons
Kidney Function Loss (GFR decrease): Age 20 to 80
| Group | Age 20 | Age 40 | Age 60 | Age 80 | Total % Loss (20–80) |
| Men | 107 | 108 | 89 | 74 | 30.8% |
| Women | 109 | 108 | 88 | 71 | 34.9% |
Conclusion: So 60 or 80 ml/min is 100% function – i.e. it is normal, not CKD, and not a disease at all.
In short: Creatinine is the marker (of toxin levels in the blood); GFR is the meaning. One is a substance we measure, and the other is a calculation that tells us what that measurement implies for your kidney function, and health.
Monitoring both is the most effective way to catch kidney issues early, often before you even feel physical symptoms.
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