Home Forums Other Specialities Nephrology/Urology RENAL FUNCTION TESTS INCLUDING GFR

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    The kidneys (right and left) perform several important functions. The functional unit of the kidney is the nephron which is made up a glomerulus (the filtration unit) and the convoluted renal tubules into which the substances filtered out by the glomerulus enter and are removed from the body as urine

    • Removal from the body of toxins and waste products formed in the blood via the urine
    • Proteins should not be excreted and should be retained in blood
    • Maintaining water and electrolyte balance
    • Maintenance of blood pressure
    • Secrete hormone erythropoietin that stimulates formation of red cells in the bone marrow

    If the kidneys aren’t functioning properly, they are unable to remove toxins such as urea and other waste from the blood which build up and substances like protein, that should be retained are filtered out, causing several complications

    Several urine and blood tests are available to measure kidney function

    • Urine dipstick
    The dipstick is a chemically treated test strip that is dipped into a urine sample. Dipsticks are readily available and can quickly give information about urine abnormalities such as presence of red cells (that may not visible to the naked eye), protein, sugar, ketones, pus cells, bacteria. Using the dipstick the degree of proteinuria, red cells, sugar etc can be graded as 1+, 2+, 3+ indicating the severity

    Based on the dipstick results, the doctor may decide to perform further blood tests as appropriate to rule out certain conditions.

    • Urine protein
    When the dipstick shows 1+ or more protein, the protein may be quantified accurately by a test called albumin-creatinine ratio (ACR)

    • Microalbuminuria
    This test detects very small amounts of albumin which may not be detected by routine dipstick tests. It is typically used in conjunction with a creatinine test to give an albumin-to-creatinine ratio, and useful early stage kidney disease especially in patients with diabetes and hypertension

    • Urine microscopy
    Microscopic examination of the urine may reveal red cells, presence of bacteria, and salt crystals (may indicate kidney stone) and other abnormalities which point to a specific diagnosis.

    • Urine specific gravity and osmolality
    Specific gravity of urine may be measured in some cases. It indicates how well the kidneys are able to dilute or concentrate the urine. It is normally between 1.002 and 1.030.

    Urinary osmolality measures the number of dissolved particles per unit of water in the urine. It is more accurate than specific gravity to measure urinary concentration.

    ? Serum creatinine
    Creatinine is a waste product formed due to muscle wear and tear and is excreted in the urine. A creatinine level of more than 1.2 in women and higher than 1.4 in men may be an early sign that the kidneys are not functioning properly. As kidney disease progresses, the level of creatinine in the blood rises. Creatinine levels in the blood also depends on age, race and body size.

    ? Calculated creatinine clearance
    This is measured as an indicator of GFR. This involves a 24-hour urine collection or preferably over an accurately timed sample over a 5 to 8 hour period since 24-hour collections are not always reliable. Creatinine clearance is then calculated using the formula

    C = (U x V) / P where C = clearance, U = urinary creatinine concentration, V = urinary flow rate (volume/time ie ml/min), and P = plasma creatinine concentration

    ? Blood Urea Nitrogen (BUN)
    The blood urea nitrogen (BUN) test measures the amount of nitrogen in the blood. Urea nitrogen is formed from the breakdown of protein in the body and are excreted by kidneys. These will therefore accumulate in the blood and become elevated if renal function is impaired.

    • Glomerular filtration rate (GFR) or creatinine clearance is the amount of plasma that is filtered through the glomerulus into Bowman’s space over a given time period and is an important measure of renal function. It is usually expressed as ml/min.
    • Clinicians commonly rely on creatinine clearance values to measure GFR. Creatinine is a daily endogenous product predominantly derived from creatine phosphate metabolism within muscle
    • As stated above, GFR is routinely done as part of investigations to check the status of renal function. If GFR value is low, then the kidneys are not functioning properly.
    • Measuring GFR can help detect kidney disease.
    • Early detection of renal impairment ensures early treatment can be initiated and complications are prevented

    A rough estimate of the GFR can be determined by using the formula:
    GFR = 140 – [ age of the person]. For example, in a healthy 30 yr old, GFR should be approximately 110mls/min.

    However, GFR is affected by patient factors such as age, sex and your body size and muscle mass.

    Therefore, many formulae used to calculate GFR but the ones used most often are the Cockcroft & Gault
    and MDRD formulae.

    The Cockcroft-Gault equation, takes into consideration age, sex and muscle mass . In men the result is multiplied by 1.23 due to increased muscle mass in males.

    • Alterations in renal blood flow
    • Glomerular capillary hydrostatic pressure
    • Glomerular capillary permeability
    • Effective filtration surface area
    • Changes in capsular hydrostatic pressure
    • Alterations in concentration of plasma proteins eg albumin
    • Size, shape and charge of molecules

    The above parameters get affected in several conditions affecting the kidneys, either directly or indirectly, and in turn influence the kidney function and GFR.

    The normal value for GFR is 90 or above. A GFR less than 60 is a warning sign of early kidney disease. A value less than 15 indicates severe renal impairment needing treatment such as dialysis or a kidney transplant.

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