Kidney Function Tests and Interpretation of Results.

Kidney function tests are group of investigations done to evaluate the function of the kidneys.
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They can be classified to two categories
A. Tests for glomerular function
B.Tests for tubular function
The tests that are part of the Kidney Function test panel are:
(a) Urine examination
(b) Serum Urea
(c) Serum creatinine
(d) Blood urea nitrogen (BUN)
(e) Calcium
(f) Phosphorus
(g) Protein
(h) Albumin
(i) Creatinine clearance
(j) Urea clearance
(k) Inulin clearance
(l) Dilution and Concentration test
(l) Serum electrolyte levels
URINE EXAMINATION
Before we do a quantitative examination of urine a qualitative examination is
necessary as it can provide excellent clues to the nature and location of the lesion
in the renal system.
This examination consists of a physical examination where the colour, odour,
quantity, specifc gravity etc of the urine is noted. Microscopic examination of
urine is done to rule out any pus cells, Rbc casts, Crystals.
SERUM UREA
Urea is the end product of protein catabolism. The urea is produced from the
amino group of the amino acids and is produced in the liver by means of the
Urea cycle.
BLOOD UREA NITROGEN (BUN)
Sometimes the Serum urea level is expressed as blood urea nitrogen. BUN can
be easily calculated from the serum urea level
A rise in blood nitrogen level is known as azotemia.
SERUM CREATININE LEVEL
Creatine is a small tripeptide found in the muscles. It stays in its phosphorylated form and releases energy for any burst of muscular activity. It is released from the muscles during regular wear and tear and is converted to creatinine (its internal anhydride). It is to be remembered that unlike urea, creatinine is not a toxic waste. It is simply used as a marker of renal function.
Creatinine is freely filtered at the glomerulus and is also to a very small extent secreted into the tubules. So any problem with gromerular filtrations has a significant effect on the excretion of creatinine resulting in a much substantial rise in serum creatinine level.
Normal serum creatinine level is 0.6 to 1.5 mg/dl. Serum creatinine is a better indicator of renal function and more specifically glomerular function than urea.
For a particular individual the creatinine level is dependent on the muscle mass
and muscle wear and tear. There may be significant difference in creatinine level of individuals with vastly differing muscle mass. For example a body builder or athlete will have higher creatinine levels than a sedentary desk worker.
Similarly creatinine level will also increase in case of any muscle trauma or excessive wear and tear as seems in athletes and people involved in hard physical labor.
Creatinine is most commonly measured in laboratories calorimetrically by Jaffe’s method.
UREA CLEARANCE
Urea clearance is the hypothetical amount of blood from which kidney clears
urea in one minute. This is measured by measuring the concentration of urea in blood, concentration of urea in urine and amount of urine excreted over a one hour interval.
Urea clearance is less than its glomerular filtration as some of the urea that is filtered at the glomerulus is reabsorbed at the tubules.
To measure urea clearance first the patient is made to void urine and then the made to drink two glasses of water. Then the urine is collected after an hour and a blood specimen is also collected at the same time. Then the patients urine sample is collected after another hour. The urea level in the two urine samples and the blood sample is measured. The urine volume is calculated as urine output per minute.
The functional unit of the kidney is called a nephron. It consists of two main
parts, the glomerulus and the tubular system.
The glomerulus is composed of a bowman’s capsule and a tuft of leaky blood
vessels encapsulated by the bowman’s capsule. The primary purpose of the
glomerulus is filtrations. The leaky vessels filter into the glomerulus almost all
the water, electrolytes, small proteins, nutrients such as sugar