TEST FOR DETECTION OF UROBILINOGEN IN URINE
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Fresh urine sample should be used because on standing urobilinogen is converted to urobilin, which cannot be detected by routine tests. A timed (2-hour postprandial) sample can also be used for testing urobilinogen.
Methods for detection of increased amounts of urobilinogen in urine are Ehrlich’s aldehyde test and reagent strip test.
1. EHRLICH’S ALDEHYDE TEST
Ehrlich’s reagent (pdimethylaminobenzaldehyde) reacts with urobilinogen in urine to produce a pink color. Intensity of color developed depends on the amount of urobilinogen present. Presence of bilirubin interferes with the reaction, and therefore if present, should be removed. For this, equal volumes of urine and 10% barium chloride are mixed and then filtered. Test for urobilinogen is carried out on the filtrate. However, similar reaction is produced by porphobilinogen (a substance excreted in urine in patients of porphyria).
Take 5 ml of fresh urine in a test tube. Add 0.5 ml of Ehrlich’s aldehyde reagent (which consists of hydrochloric acid 20 ml, distilled water 80 ml, and paradimethylaminobenzaldehyde 2 gm). Allow to stand at room temperature for 5 minutes. Development of pink color indicates normal amount of urobilinogen. Darkred color means increased amount of urobilinogen (Figure 818.1).
Since both urobilinogen and porphobilinogen produce similar reaction, further testing is required to distinguish between the two. For this, Watson-Schwartz test is used. Add 1-2 ml of chloroform, shake for 2 minutes and allow to stand. Pink color in the chloroform layer indicates presence of urobilinogen, while pink coloration of aqueous portion indicates presence of porphobilinogen. Pink layer is then decanted and shaken with butanol. A pink color in the aqueous layer indicates porphobilinogen (Figure 818.2).
False-negative reaction can occur in the presence of (i) urinary tract infection (nitrites oxidize urobilinogen to urobilin), and (ii) antibiotic therapy (gut bacteria which produce urobilinogen are destroyed).
2. REAGENT STRIP METHOD
This method is specific for urobilinogen. Test area is impregnated with either p-dimethylaminobenzaldehyde or 4-methoxybenzene diazonium tetrafluoroborate. Also read: URINE STRIP TEST — UNDERSTANDING ITS LIMITATIONS.