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Introduction

The kidney has a number of functions, including:

  • Maintenance of homeostasis.

  • Production of erythropoietin.

  • Maintenance of blood pressure.

  • Hydroxylation of vitamin D.

It is the investigation of the first of these with which this Chapter is concerned.

Creatinine as an indicator of renal function

One of the major roles of the kidney is the excretion of soluble waste. This is achieved by the process of glomerular filtration. Creatinine is commonly used as an indicator of the glomerular filtration rate (GFR). Creatinine is a non-toxic breakdown product of creatine, a short-term energy store found in muscle (in the form of phosphocreatine) (1.1).

1.1

Metabolism of phosphocreatine to creatinine.

Laboratory analysis of serum creatinine entails chromo-graphic analysis of the products of the Jaffe reaction. Creatinine is mixed with alkaline picramate and an orange coloured compound is formed (1.2). The product is then analysed by a colorimeter.

1.2

Creatinine after alkaline picramate reaction.

Table 1.1 shows possible causes for a raised creatinine.

Table 1.1Causes of a raised creatinine

The 'goulash effect' is a transient rise in serum creatinine after ingestion of large quantities of boiled meat.

Glomerular filtration rate

Measurement of serum creatinine has its limitations. The relationship between serum creatinine and GFR is not linear, and serum creatinine level may not rise significantly until GFR is as low as 30% of normal.

GFR is an important measurement that provides clinicians with information about overall renal function. GFR can be estimated in a number of ways.

Clearance concept

GFR can be estimated by measuring clearances of a substance in which filtration by the glomerulus equals excretion in the urine. Clearance is the volume of plasma that is completely cleared of a substance by the kidneys per unit time (1.3).

Calculation of creatinine clearance

Creatinine is an endogenous substance that is present at a relatively constant level (1.4). Measurements and calculation according to the formula in figure 1.3 require 24-hour collection of urine (1.5). Therefore, patients need to be compliant for results to be accurate.

1.4

Calculation of creatinine clearance (ml/min).

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