Chromogens, urinary

Plasma and urinary creatinine are commonly measured by the colorimetric alkaline picrate method of Jaffe or by alternative enzymatic methods. Enzymatic methods use creatinine amidohydrolase or creatinine iminohydrolase and are more specific for creatinine. The measurement of plasma creatinine may be affected by endogenous noncreatinine chromogens (e.g., bilirubin and ketones) this can overestimate plasma creatinine in dogs by up to 45% and to an even greater extent in rats... 

This consists of p-dimethylaminobenzaldehyde in hydrochloric acid. It is used in the estimation of urinary urobilinogen (which gives a red colour), porphobilinogen, (giving a red colour which can be extracted into organic solvents) and indican (when the red chromogen can be extracted into alkali). 

In addition to the 17-ketosteroids, urinary extracts contain materials possessing significant absorption in the region of 520 mu when treated with the reagents used in the Zimmermann reaction. It is these so-called urinary chromogens which complicate the estimation of the 17-ketosteroids. 

The advantages of this method are (/) there is a linear relationship between the concentration of ketosteroid and optical density over a wide range of steroid concentration, and (2) a correction may be applied for chromogens which interfere in this analysis. The major drawback of this procedure is the instability of the ethanolic potassium hydroxide solution which must either be prepared freshly every few days or suitably preserved (58,131). It should also be noted that the major urinary ketosteroids do not have equal extinction coefficients under the conditions proposed by Callow. Etiocholane-3a-ol-17-one gives significantly more color than an-drosterone, epiandrosterone, or dehydroepiandrosterone (93,130). 

Readings of optical density at two different wave lengths may be used to apply the color correction equation of Gibson and Evelyn (55) which corrects for the enhancement of absorption due to the urinary chromogens. The validity of this equation has been confirmed by Engstrom and Mason (47). Fraser et al. (51) have applied the equation extensively and it has now become a standard part of this method. The modified Callow method has been adopted as the standard procedure by the British Medical Research Council (90). Mason (86) has suggested its adoption in the United States. 

A correction may be applied for the interference caused by urinary chromogens. 

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