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The principle of determination of proteins in urine using diagnostic strips is based on the so-called protein error of an acid-base indicator, eg tetrabromophenol blue , tetrabromophenolphthalein ethyl ester or 3´, 3´´, 5´, ​​5´´-tetrachlorophenol-3,4,5,6- tetrabromosulfophthalein. Like any acid-base indicator, these substances change color at a certain pH (they behave like weak acids, while the protonated form has a different color than the dissociated form): at pH below 3.5 they are yellow, at higher pH they are green to blue. In addition to the indicator, there is a buffer in the reaction zone of the test strip, which maintains the pH in the range of 3.0 to 3.5, so the indicator is yellow. If there are proteins in the sample, they bind to the indicator with their amino groups. However, this changes its properties - the transition area shifts towards a more acidic pH. This means that at the stated constant pH between 3.0 and 3.5, the protein-bound indicator will be green, as if it were in a more alkaline environment (hence the protein error of the indicator ).). The color intensity depends on the protein concentration, varies from green to blue and is evaluated visually or instrumentally.

In highly alkaline urine (pH above 8) or if the urine is very concentrated, the test may give false positive results (buffer will be depleted in the reaction zone). In these cases, acidify the urine with a few drops of dilute acetic acid to pH 5-6 and repeat the test. False positives can also be caused by high concentrations of some substances with amino groups (contamination of the sampling vessel with some disinfectants), which bind to the indicators similarly to proteins.

The disadvantage of the test strips is their different sensitivity to individual proteins. The strips react very well with albumin and indicate its presence in urine from 0.1 to 0.5 g / l. They are significantly less sensitive to globulins, glycoproteins and Bence-Jones protein. These diagnostic strips do not show an increase in albuminuria to values ​​up to about 200 mg / l, resp. daily albumin losses in the range of 30 to 300 mg / 24 hours, which accompanies especially the earlier phases of some nephropathy. Immunochemical methods can be used to screen for increased albuminuria, such as special diagnostic strips based on immunochromatographic principles or immunoturbidimetry.