Introduction

Kidneys are vital organs responsible for filtering out waste substances through urine excretion. Healthy kidneys can produce an average between 1 – 2 litres urine per day. The kidneys have ultrafilters called glomerulus. These glomerulus filter blood circulating the body and allow only waste substances known as urea and water to exit into urine.

Inability to filter out substances will result in having protein, glucose and even blood in the urine volume. Early detection of kidney disease is crucial to further design method of treatment in patients. One of the referred tests to measure kidney function is estimation of glomerular filtration rate (eGFR).

Definition

Estimation of glomerular filtration rate (eGFR) is a test to determine level of kidney or renal function. This test use blood samples from designated patients. eGFR is actually the rate in which blood is filtered by glomerulus each minute. The rate is calculated according to age, body size and gender of patients respectively. The lower eGFR calculation means the higher tendency for patients to have chronic kidney disease.

Intended Use

This test has been designed to determine kidney function level, especially in chronic kidney disease patients. Other than that, this test is also recommended for patients with diabetes, those who frequently gets urinary tract infections, heart disease patients, those with high blood pressure and those having urinary blockage. These recommendations are due because of the challenges these diseases impose on kidney function. Other risk factors for kidney disease include family history, older age, ethnic group and smoking. Patients with prescribed nephrotoxic drugs (drugs that can destroy kidney cells) is also monitored using eGFR test.

Once results are determined, doctors can plan out the best way of treatment for patients. The sooner kidney dysfunction is diagnosed and treated the greater odds of preserving remaining nephrons, and preventing the need for dialysis.

Parameters

There are several ways to measure eGFR. Clinically, the referred test is to measure serum creatinine level. Blood sample is drawn through a needle from a vein in patients’ arm to calculate eGFR. Sometimes patients are also acquired to collect 24-hour urine sample. This other test is to calculate creatinine clearance which is excreted in the urine throughout the day. Patients need to collect urine from first empty bladder of the first morning until the last full bladder of the next morning.

Creatinine is a waste product of creatine phosphate (from muscles) and flows freely in the blood. These waste products will be filtered out by glomeruli and excreted in urine. Renal tubule will also excrete creatinine, making the levels of creatinine clearance raised 10-20% higher than levels in the serum.

Reference Intervals

Chronic kidney disease is categorized into six stages:

*This classification may mislabel patients with mildly reduced kidney function, especially the elderly, as having a disease.

Advantages

Estimation of eGFR is the best index of renal function. The clearance of inulin is accepted standard for the measurement of GFR. However in clinical studies, endogenous creatinine clearance best approximate the inulin clearance. Creatinine clearance has become the preferred method opposite of inulin calculation. Inulin, or the inulin-analog sinistrin are neither reabsorbed nor secreted by the kidney after glomerular filtration. Their rate of excretion is directly proportional to the rate of filtration of water and solutes across the glomerular filter. However, inulin is not a natural product of the body and need to be injected into patients. The extra step needed to complete eGFR measurement using inulin is seen as a hassle both for clinicians and patients, hence opted only when necessary.

Limitations

Some medications may affect serum creatinine level, resulting in false positive result. These include antibiotics and stomach acid medicines. Care providers are must consult patients on any medication taken prior to recommending eGFR test.

The test eGFR is also affected by pregnancy, malnourishment, obesity, high percentage of meat diet and also vigorous exercise before testing. Patients need to know how their lifestyle and activities might affect the results of their eGFR test, and a little alteration in the diet and physical activity prior to the test itself might give them better eGFR measures.

Incomplete urine specimen will also give false results. Hence, this test is seldom referred to and only recommended when specific amount of eGFR need to be measured.

Taking a blood sample using a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. Patients may feel a slight stinging sensation or pain. The site which blood was drawn may be slightly sore afterward.

Acceptance in Clinical Settings

Clinically, serum creatinine level and 24-hour urine creatinine clearance are widely accepted and referred to as the test taken by patients suspected of chronic kidney disease. However, if a more accurate test is needed to estimate glomerular filtration rate, then patients can go through inulin-injection method.

References

1. Guyton, A. & Hall, J. (2006). Urine formation by the kidneys : glomerular filtration, renal blood flow, and their control. In Gruliow, Rabecca (Ed), Textbook of medical physiology (11th ed.) (pp 308-325), Philadelphia, Pennsylvania : Elsevier Inc.
2. Schwartz, G.J., Haycock, G.B., Edelmann, C.M., & Spitzer, A. (1976). A simple estimate of glomerular filtration rate in children derived from body length and plasma creatinine. Paediatrics, 58(2), 259-263.
3. Stevens, L.A., Coresh, J., Greene, T., & Levey, A.S. (2006). Assessing kidney function – measured and estimated glomerular filtration rate. The New England Journal of Medicine, 354 (23), 2473-2483.
4. Gault, M.H., Longerich, L.L., Harnett, J.D., & Wesolowski, CD. (1992). Predicting glomerular function from adjusted serum creatinine. Nephron, 62 (3), 249-256.
5. Lai, S.H., Loke, M.O., Ghazali, A., Bavanandan, S., Noor A. A., Balkish, M.N., Wan, N.W.M., & Muhammad, F.M.Y. (2013). A population-based study measuring the prevalence of chronic kidney disease among adults in West Malaysia. Kidney International, 84, 1034-1040.
6. Bauer, C., Melamed. M.L., & Hotstetter, T.H. (2008). Staging of chronic kidney disease : time for a course correction. American Society of Nephrology, 19(5), 844-846.
7. Eckardt, K-U., Berns, J.S., Rocco, M.V., & Kasiske, B.L. (2009). Definition and classification of CKD : the debate should be about patient prognosis-a position statement from KDOQI and KDIGO. American Journal of Kidney Diseases, 53 (6), 915-920.