Before we discuss Kidney Function Tests, in medical terms Renal Function Tests (and it is always a good practice to use appropriate term instead of layman’s term), it is important to know why do we perform these tests. As we know kidney is one of a vital organ of human
body and performs at least 4 functions:

1- Excretory Functions: Helps in excretion of waste products and drugs.

2- Regulatory Functions: Controls body fluid volume and composition.

3- Endocrine Functions: Produces erythropoietin, rennin and prostaglandins.

4- Metabolic Functions: Metabolizes Vitamin D and small-molecular-weight proteins.

So, now we know the real importance of a healthy kidney. But what if
something wrong happens to this vital organ, then by which means we are
going to assess its functions?

The answer lies in the renal function
tests, which tell us about the overall status of kidney’s functions.

There are 3 major renal function tests, which we would be discussing
below one by one:

1-Glomerular Filtration Rate (GFR)

The GFR provides a useful index of overall renal function. It measures the amount of plasma ultra filtered across the glomerular capillaries and correlates well with the ability of the kidneys to filter fluids and various substances. Daily GFR is in the range of 100-120 ml/min.A low GFR is an evidence of serious progressive renal disease and indicates a decrease in total functioning renal mass. The GFR reduces in both acute and chronic renal failure.GFR can be estimated by the creatinine clearance with the help of following formula:

C=U multiplied with V/P where:

C: Creatinine Clearance

U: Urinary Clearance mg/dl

P: Plasma Creatinine mg/dl

V: Urine Flow Rate ml/min

Decreased GFR manifests as raised serum urea and creatinine. However serum urea and creatinine are not elevated above the normal range until there is a reduction of 50-60% in the GFR. Therefore, it should be noted that normal urea and creatinine levels do not rule out renal
insufficiency and when serum urea and creatinine begin to rise, more than 50-60% of renal damage has occurred.More accurate measurement of GFR is now most easily undertaken by ascertaining the clearance of 51Cr-labelled ethylenediamine-tetracaetic acid (EDTA).

2-Serum Urea

Urea is produced in the liver and is the end product of protein catabolism. Urea is freely filtered by the glomerulus and about 30-70% is reabsorbed in the nephrons. It is very important to note that dehydration causes increased urea reabsorption. Therefore urea level increases during dehydration as in case of acute diarrhea while the renal function is preserved and creatinine is normal. A normal urea to creatinine ratio is 20:1.This ratio is increased in dehydration while
the ratio remains same in renal insufficiency. There are also some other factors that affect serum urea level such as following:

Increased serum urea independent of GFR

• Dehydration
• Catabolic state
• High protein diet
• Glucocorticoids
• Tetracyclines

Decreased serum urea independent of GFR

• Liver disease
• Malnutrition
• Low protein diet
• Old age

3-Serum Creatinine

Creatinine is by far the most useful clinical test in assessing progression of renal failure. However, at least 50% of renal function is lost before rising serum creatinine. Therefore, normal level of creatinine does not rule out impairment of renal function.

Serial estimation of serum creatininine provides the best indication of state of renal function in patient with Chronic Renal Failure (CRF).

Creatinine is actually the product of muscle metabolism .It is freely filtered and not reabsorbed, however small amount is eliminated by tubular secretion that increases with dehydration overestimating the GFR.

As in case of serum urea, there are factors that affect serum
creatinine which are mentioned below:

Increased serum creatinine independent of GFR

• Ketoacidosis
• Drugs like Cephalothin, Cefoxitin, Aspirin, Cimetidine, and
  Trimethoprim

Decreased serum creatinine independent of GFR

• Advanced age
• Liver disease
• Cachexia