How is kidney function measured

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When the kidneys are no longer working efficiently, tell-tale signs appear in the blood and the urine - but not until a certain stage. In the

Figure 2.3 Scanning electron microscopy shows the various large pores in the basal membrane, through which the blood is filtered. [3]

Figure 2.3 Scanning electron microscopy shows the various large pores in the basal membrane, through which the blood is filtered. [3]

Electron Microscopy Kidney

blood, for example, creatine and urea accumulate. Creatine is a waste product of muscle metabolism that is normally excreted through the kidneys. Healthy people (Table 2.1) have a creatine serum concentration of between 0.6 and 1.2 mg/100 ml. The normal value in an individual depends on the muscle mass. In someone with less muscle, it is lower; in 'muscle men', it is higher. If kidney function fails, the creatine concentration in the blood gradually rises.

Generally, though, mild kidney impairment does not lead to a noticeable change in serum creatine concentration. It rises only when kidney function has decreased by more than half. In people with little musculature, the low starting point for the creatine concentration can mean that it stays within the normal range for a long time, even when kidney function is seriously compromised.

A more exact picture of kidney function is given by the so-called creatine clearance rate. To measure this, the urine must be collected for 24 hours or overnight, then the creatine concentration in the blood and in the urine is determined. A formula can then be used to calculate the filtration rate and thereby the activity of the kidney. However, a precise value is obtained only when all the urine is collected. In practice, this is where mistakes can easily be made.

A simpler and faster method is to calculate the creatine clearance using a formula based on serum creatine concentration, body weight and age (see the example on page 119). This gives reasonable values for a wide range of kidney function. Normally, the rate of creatine clearance lies between 80 and 140 ml/min.

Urea is the end product of protein metabolism and is excreted through the kidneys. In healthy people, the concentration of urea in the blood is less than 45 mg/100 ml. As

Table 2.1

These blood parameters give information about the activity of the kidney

Parameter Normal range


Creatine clearance




Potassium Uric acid

80-140 ml/min

Less than 45 mg/dl Less than 7 mmol/l

Less than 7 mg/dl Less than 415 imol/l

Warning: the normal values vary according to the laboratory method used to measure them and are partially dependent on age and sex.

kidney function fails, the amount of urea in the blood rises. This occurs particularly when a person eats a lot of protein. The concentrations of the blood salts, potassium, calcium and phosphorus, as well as that of uric acid, can also give information about the state of the kidneys.

In the urine, the most important marker for impaired renal function is protein excretion. Normally, only a very small amount of albumin, a special blood protein, is found in the urine - less than 20 mg/l, which is too little for the standard tests to detect. If the albumin concentration rises above a certain threshold, this can be one of the first signs of kidney failure. Protein may occur in the urine as a result of other circumstances, such as after major bodily damage, infection, pressure on the kidneys or low temperatures.

Examination of the urine composition can also give clues regarding the state of the kidneys. Usually, no red or white blood cells should be visible in the urine under a microscope. If they are present, it is an indication that something is wrong with the kidney-bladder system. Such a finding should be the starting point for further investigations.

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  • eyob
    How is kidney function measured?
    2 years ago

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