What are the limitations of a creatinine blood test?
What is a normal creatinine level?
60-120 mcmol/L (micromoles per litre). It is used to assess kidney function – the higher the number, the worse the function.
What is creatinine?
Creatinine is a metabolic product of creatine and phosphocreatine, both of which are found almost exclusively in muscle.
Thus, creatinine production is proportional to muscle mass and varies little from day to day.
Age- and gender-associated differences in creatinine production are largely attributable to differences in muscle mass.
The creatinine molecule is small (molecular weight 113 Da), does not bind to plasma proteins, and is freely filtered by the glomerulus. As the production is constant, the serum creatinine level depends on the rate of clearance, which mainly reflects the GFR.
However, it has long been appreciated that creatinine is also secreted by the renal tubule, that is blocked by some commonly used medications including cimetidine and trimethoprim.
So what are its limitations?
It has several. For example, women usually have lower creatinine levels than men because women, on average, have less muscle than men.
Creatinine level does not just depend on someone’s kidney function. It is also affected by your:
- Size and weight – creatinine higher in larger people
- Muscle mass and use – higher in people with bigger muscles that are used
- Exercise
- Race – higher in Black people
- Gender – higher in men
- Fluid state on the day of the test – if dehydrated can be falsely increased
- How the chemical test is done, on the day of the test – it is not that accurate. So if measured twice on the day of the test (or the same sample measured twice), it can be upto 10% different
- Diet
- Tablets – some tablets like cimetidine and trimethoprim falsely can falsely elevate the test.
This is why a single measurement does not have a lot of meaning. It has to be repeated; and then the results interpreted by a doctor seeing you and taking the above factors into account. The trend in creatinine level is more important than a single measurement.
For example a blood creatinine level of 125 mcmol/L (i.e. just above normal) can indicate Stage CKD4 in a frail elderly White lady, but is normal for a muscular young Black man.
Summary
We have described the limitations of a creatinine blood test. It needs to be interpreted with the patient in front of you, and by comparison to previous levels. We hope it has been helpful.
Other resources
What is a normal creatinine?
What are the limitations of a GFR?
This is a good review article: Bargnoux, 2018
Predictive value of normal creatinine at end of AKI stay (Efros, 2023)
Last Reviewed on 14 June 2024