hepatic excretion of conjugated bilirubin, resulting in increased amounts of urobilinogen that in turn causes an increase in reabsorption, recirculation and renal excretion. The reactions that take place in the test strip vary according to the manufacturer, but in reality there are two reactions that are most frequently used. Some manufacturers use Ehrlich's reaction (1), in which urobilinogen reacts with p-dimethylaminobenzaldehyde (Ehrlich's reagent) in order to produce colours that vary from light to dark pink. Other manufacturers use a diazo coupling reaction (2) that uses 4-methoxybenzene-diazonium-tetrafluoroborate to produce colours that vary from white to pink. The latter reaction is more specific. (1) Reaction on Multistix (in acid medium) Urobilinogen + p-dimethylaminobenzaldehide → Red dye (2) Reaction on Chemstrip (in acid medium) Urobilinogen + 4-methoxibenzene-diazonium-tetrafluoroborate → Red azo dye A number of substances interfere with the Ehrlich reaction on the Multistix strip: porphobilinogen, indican, p-amino salicylic acid, sulphonamide, methyldopa, procaine and chlorpromazine. The test should be carried out at room temperature as the reaction's sensitivity increases with temperature. Poorly stored samples can yield false negative results as the urobilinogen suffers photo oxidation to urobilin that does not react. The formaldehyde used as a preservative produces false negatives in both reactions. Urinary infections Urinary infections can be identified including bacteriuria and pyuria. Nitrites test The test for nitrites is a rapid screening method for possible asymptomatic infections caused by nitrate-reducing bacteria. Some of the gram negative bacteria species that most commonly cause urinary tract infections (Escherichia coli, Enterobacter, Klebsiella, Citrobacter and Proteus) have enzymes that reduce the nitrate present in urine to nitrite. The test is a rapid screen for possible infections by enteric bacteria, but it does not replace the urinalysis tests nor microscopic examination as diagnostic tools, nor subsequent monitoring as many other microorganisms that do not reduce nitrate (gram positive bacteria and yeasts) can also cause urinary infections. The reactive strips detect nitrite by using the Griess reaction in which the nitrite reacts in an acid medium with an aromatic amine (para-arsanilic acid or sulphanilamide) in order to form a diazonium salt that in turn reacts with tetrahydrobenzoquinoline to produce a pink azo dye. 1) In an acid medium Para-arsanilic acid or sulphanilamide + → Diazonium salt 2) In an acid medium Diazonium salt + tetrahydrobenzoquinoline → Pink azo dye The nitrite test is not particularly reliable and negative results in the presence of clinical symptoms are not uncommon,