Thursday, 10 September 2009

Q & A: Could you explain the good, medium and low eGFRs and what they mean?

Q: My transplanted kidney is now in its 15th year and as far as my blood figures show it is still doing fine. My eGFR is 42 if that is anything to go by. I am coming up to my 87th birthday. could you put in the next Kidney Life some information on the eGFR to explain to members (readers) the good, medium and low eGFRs and what they mean? Best wishes,
Frank Wood

A: Congratulations Frank, on your upcoming 87th birthday and the 15th year anniversary of your kidney transplant. Your experience of receiving a transplant in your early 70s and continuing to live a long and productive life will be a great inspiration to others with kidney disease. Thanks for your question about eGFR and what it means. eGFR stands for estimated Glomerular Filtration Rate. Glomerular Filtration Rate is now widely recognised as the best measure of kidney function. It can be estimated from the level of creatinine in the blood and since a normal GFR in a young adult is about a hundred mils per minute, the eGFR can be thought of as percentage kidney function. Talking about eGFR as percentage kidney function has been a useful way to demystify kidney disease for doctors, nurses and patients. However, eGFR isn’t a perfect measure of kidney function, it has several limitations and a single figure will never tell you everything you need to know about an individual’s kidney or transplant function.

Each kidney is made up of about 1 million nephrons. The nephrons do the work of the kidney – getting rid of waste products, controlling and adjusting the blood chemistry and producing the metabolic messengers vitamin D and erythropoietin that keep our bones and blood healthy. Nephrons do this by filtering the blood through tiny specialised blood vessels called glomeruli and then fine tuning what the body needs to retain or be eliminated along the tubules which make up the rest of the nephron. Filtration of blood across these filters is therefore the first and most important stage of cleansing the blood of impurities. This is called Glomerular Filtration Rate or GFR. The GFR can be directly measured by using radioactive isotopes or by infusing certain inert chemicals into the bloodstream. This is sometimes done to very precisely measure kidney function in people offering kidneys for live donation. Such tests are not practical in routine clinical practice for day to day assessment of kidney function. Fortunately, GFR can be estimated from serum creatinine which is easily measured. Creatinine has been used to give an idea of kidney function for many years. Creatinine, used alone, is an insensitive marker of kidney function, up to 50% of kidney function can be lost before the serum creatinine concentration rises above the normal range. Estimated GFR is a better measure of kidney function that serum creatinine. Since 2006, every laboratory in the UK has calculated a standardised eGFR using a formula based on sex, ethnicity, age and serum creatinine.

Routine reporting of eGFR was one of the recommendations of the National Service Framework for Renal Services. It has allowed early identification of kidney disease in literally millions of people. Early investigation and treatment of kidney problems has become a core part of general practice.

All people with kidney disease are now entitled to a care plan designed around their individual needs to keep kidney function as good as possible for as long as possible and to reduce cardiovascular risk. We are already starting to see improvements in blood pressure control in general practice and earlier referral to the multi professional kidney team to support individuals in making decisions about transplantation and dialysis when replacement therapy is needed. The eGFR is also the way chronic kidney disease is staged and monitored over time. Staging and monitoring also requires assessment of proteinuria by measurement of the albumin creatinine ratio, or ACR, in the urine. Proteins in the urine, of which albumin is the main protein, indicate a higher risk of progressive kidney disease and of cardiovascular risk.

Thankfully, protein in the urine can be treated by certain drugs including the ACE inhibitors and ARB class of blood pressure drugs. So regular, accurate assessment of eGFR and ACR are absolutely essential for people with kidney disease. The eGFR tells GPs when specialist kidney referral is needed – when the eGFR falls below 30 mls per minute. It helps the patients, nurses and doctors in getting the timing of transplantation before dialysis right (at 15 mls per minute or below) and/or in the preparation for dialysis such as when to operate to create a fistula for haemodialysis. Looking at an individual’s eGFR over time tells you if the kidney function is stable, improving or declining – it acts as a trigger for further evaluation and changes in treatment as well as providing a window on the future. But eGFR doesn’t tell you everything and used alone can be misleading.

The eGFR needs to be interpreted cautiously at the extremes of age and extremes of body mass, it can be interfered with by some drugs, it is not always helpful in the acute situation and the accuracy of the equation used hasn’t been proven in all ethnic minority groups and clinical situations including transplantation. It is not a perfect measure of kidney function, rather it provides a guide.

Even eating a meal before the measurement can affect the reported level. There are equations for using children but the routine laboratory based equation is not suitable for people under the age of 18 years. It’s of no value in people receiving dialysis. Hence the need to look at more than one thing to get the right answers – the ABC of kidney disease is know your: A ACR – albumin creatinine ratio in the urine; B blood pressure – aim for perfect blood pressure control; C creatinine for the eGFR = percentage kidney function

The introduction of eGFR into the National Health Service has helped demystify kidney disease. It has promoted the understanding of kidney disease by doctors and nurses. I would encourage all people with kidney disease, or at risk of kidney disease, to know their eGFR and to keep track of how it is changing. Your eGFR should be discussed as part of your care planning. People with kidney disease and the healthcare professionals who care for them need to speak the same language – so you need to know your eGFR, your ACR and your blood pressure.

published in Kidney Life, Autumn issue 2009