Get up to date with changes in kidney blood testing
Dr Ridwan Ahmed
Dr May Aldean
With collaboration from Dr Kathryn Griffith
Royal College of General Practitioners (RCGP) Clinical Champion for Chronic Kidney Disease (CKD)
The NICE CKD 2014 guidelines now recommend the use of the CKD Epidemiology Collaboration creatinine (CKD-EPI) equation to estimate the glomerular filtration rate (GFR). Most laboratories until now have used the Modification of Diet in Renal Disease (MDRD) formula to automatically estimate the GFR. Several studies have shown that the MDRD equation systematically underestimates the GFR, particularly in low-risk patients with a high-normal serum creatinine level. This results in the labelling of people with CKD who do not have significant kidney disease.
The CKD-EPI equation used the same variables as the MDRD equation. However, the source studies for the CKD-EPI formula include both high risk and low risk populations. This makes the CKD-EPI formula more generalizable and more accurate compared with the MDRD equation, particularly, in the relatively high ranges of eGFR (CKD stages one and two).
Although the NICE Guideline supports the introduction of the new formula, the date in which laboratories are changes to, report this has not yet been agreed by NHS England, and you are not recommended to manually calculate this at present.
Why do we need another test?
Despite the CKD-EPI formula being more accurate than MDRD these creatinine based formulas may underestimate GFR. This is specifically true in individuals with normal or near-normal creatinine values. This is likely to lead to an overestimation of people with CKD 3. There are also other factors, such as muscle mass and medications which influence the level of creatinine, not just kidney function.
The NICE guidelines therefore advise the use of eGFRcystatinC to confirm the diagnosis of CKD in certain cases, along with guidance when not to diagnose CKD.
Cystatin C is present in all nucleated cells and not influenced by muscle mass or medication. It is therefore a better marker of kidney function and prognosis. It has been used to derive an estimating equation from the same population as the CKD and reduce unnecessary patient anxiety. At present the assay is not available in all laboratories. Although it is more costly than creatinine it is estimated that reduced medical costs of over diagnosis will make it cost effective. NHS England is developing a plan for the widespread availability of cystatin C.
For further details, please click here to read the NICE guidelines.
For further details on the work Dr Kathryn Griffith and her team are doing regarding CKD as one of the RCGP Clinical Priorities, please click here.