Assessing Renal Function

Also see Abbreviated MDRD eGFR Calculator.

The kidney has several functions including the excretion of water, soluble waste e.g. urea and creatinine, and foreign materials e.g. drugs. It is responsible for the composition and volume of circulating fluids with respect to water and electrolyte balance and acid/base status. It has an endocrine function playing a part in the production of Vitamin D and erythropoietin, and as part of the renin/angiotensin/aldosterone axis. Measurements of renal function rely on measuring, in various ways the degree to which the kidney is successful in these roles.
An assessment of renal function may be required for several reasons:

• To identify renal impairment
• To monitor disease progress
• To assess baseline measurements prior to starting treatment with certain drugs
• To monitor disease progress.
• The type of measurement of kidney function performed will be determined by the reason for assessing renal activity.

Urinalysis

• Appearance - blood , colour, turbidity.
• Specific gravity - sticks measure ionic particles only, not glucose.
• pH - normally acidic, except after a meal.
• Glucose - the presence of glucose in urine may indicate increased blood glucose, or tubular disorder.
• Proteinuria - the presence of protein in the urine may be caused by, glomerular leak, raised serum low molecular weight proteins, Bence Jones proteins, myoglobulin, or protein of renal origin.
• Microscopy - UTI will show polymorphs with no casts, acute glomerulonephritis will show cells and casts, chronic glomerulonephritis shows little sediment.

Glomerular filtration rate (GFR)

This is the most frequent test of renal function. GFR varies as a function of normal physiology as well as disease. Its measurement is based on determining the volume of plasma from which a substance is removed by glomerular filtration during its passage through the kidney, in other words, the "clearance" of that substance.

• Creatinine Clearance is often used as a rough measurement of GFR, with a timed urine collection (often 24 hrs) and a blood sample taken to measure plasma creatinine during that time period. It is limited by problems of accurate urine collection, and tends to overestimate the GFR. It is also time consuming.
• Inulin GFR is the gold standard for measurement, but is a complex procedure used only when a more accurate result is important.
• Isotopic GFR is also sometimes performed using radioactive isotopes.
• Estimated GFR. The plasma creatinine concentration (alone) is only a very rough guide to renal function.^1,2 Creatinine is produced by the muscles at a relatively constant level by the body, and the plasma concentration therefore depends on the rate of excretion by the kidneys. Levels are however affected by age, gender, ethnic group, muscle bulk, ingestion of cooked meat, malnutrition and after use of some drugs e.g. trimethoprim.

It is better to estimate the glomerular filtration rate taking some of these variables. There are several equations available (use your local lab's calculation if available - as this is likely to be more accurate as it can take into account local variations in accuracy of creatinine assays):

• In the UK the preferred calculation of eGFR uses the abbreviated MDRD equation v4 (MDRD = Modification of Diet in Renal Disease Study Group):^2,3,4

Methods for measuring GFR currently being researched include:

Cystatin C

This is a small protein produced at a relatively constant rate which is reabsorbed in the proximal tubule. It may be more sensitive than creatinine in detecting a reduction in GFR, but further studies are needed.

Iohexol

Being a single injection (plasma) clearance technique, this affords a precise measure of GFR. Iohexol is an exogenous marker that is comparable to inulin and (51)Cr-EDTA and can be measured by high-performance liquid chromatography Iohexol can accurately measure GFR using a four-point plasma disappearance curve (10, 30, 120, and 300 min) or, in most cases, a two-point disappearance time (120 and 300 min).