Renal or kidney function tests (RFTs) are tests performed to assess how well the kidneys are working.

The kidneys are paired organs, approximately the size of one’s fist located on either side of the spine below the rib cage.

Structure and Function of Kidney – In Brief
Each kidney is made up of several structures called nephrons which are the basic functional units of the kidney. Each nephron is made up of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries referred to as glomerulus.

When blood (containing salt and waste products) flows through the kidney, the glomerulus filters water and solutes out of the plasma which then pass via renal tubule finally into the collecting duct system (into which all the nephrons empty their filtrate). The urine thus formed this leaves the collecting duct system and reaches the urinary bladder via the ureters. The bladder stores the urine temporarily until the person feels the urge to pass urine.

Thus the urine is an ultrafiltrate of the of the blood plasma. Urine is normally acidic with a pH of about 6. Normally the urine will not contain blood, protein, sugar or bacteria. Presence of any of these or alteration in pH may indicate underlying disease.

Important functions performed by the kidney
• Excretion of toxic waste formed in the body
• Maintenance of salt and water balance in the body
• Role in the synthesis of vitamin D and red cell production
• Important in regulating blood pressure

Tests to Measure Renal Function
The kidneys can be affected in several conditions such as diabetes, hypertension or cancers. As a result kidney function may be compromised and renal function tests are performed to assess kidney function if the treating doctor suspects impaired kidney function.

Renal function includes simple urine and blood tests available routinely.

URINE TESTS
• 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 (moderately increased albuminuria)
This test detects very small amounts of albumin (protein occurring in the blood that is normally not filtered) which may not be detected by routine dipstick tests. It may be useful to do this test in patients who have underlying diabetes or hypertension and are at a higher risk of developing kidney disease which can thus be at detected at an early stage.

• 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.

A high specific gravity (presence of more solutes) may occur in diarrhea, vomiting, dehydration
states or decreased blood flow to the kidneys. Conversely a low specific gravity may occur when the person drinks too much water (psychogenic) or in renal failure.

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.

BLOOD TESTS
The following blood tests are used to measure renal function

• Serum creatinine
Creatinine is a waste product formed as a result of normal wear and tear on muscles of the body. 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 depends on age, race and body size.

• Creatinine clearance
Creatinine clearance test compares the creatinine in a 24-hour sample of urine to the creatinine level of the blood to determine how much waste products the kidneys are able to filter out each minute.

• Estimated Glomerular Filtration Rate (eGFR)
This test is again a measure of how efficiently the kidneys remove wastes and excess fluid from the blood. It is a calculated parameter from the serum creatinine level using age and gender, race, height and weight. It should be remembered that the GFR declines with age even in normal individuals

The normal value for GFR is 90 or above. A GFR less than 60 is a warning sign that the kidneys may not be working properly. A value less than 15 indicates severe renal impairment needing treatment such as dialysis or a kidney transplant.

• 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.

Certain medications such high doses of aspirin or certain types of antibiotics may also cause an increase in BUN values.

Normal value of BUN is 7-20 mg/dL

• Serum electrolytes
Routinely measured salts (electrolytes) include sodium, potassium, chloride and bicarbonate.
Electrolyte levels may be altered in kidney disease as well as several other conditions. These values have to be interpreted along with other tests mentioned above in kidney disease.

Renal function tests may sometimes be supplemented by imaging (ultrasound, CT scan, MRI) tests that can demonstrate structural changes, change in size and position, presence of urinary obstruction, stones or cancers. or rarely a renal biopsy when a sample of kidney tissue is obtained and analysed microscopically to identify possible cause of the renal impairment. These advanced tests provide additional and more precise information and help decide upon the line of management