Metrological Characteristics of Visual Rapid Tests

Visual rapid tests are widely used in various industries and sectors of the economy, enabling simple and inexpensive qualitative or semi-quantitative analysis. The general principle of rapid tests is based on the use of color reactions of chromogenic reagents impregnated onto various sorbents. The conditions and format of the analysis ensure a visually observable analytical signal proportional to the amount of the substance being determined. To estimate the analyte concentration in the sample, a color comparison scale is used-a computer simulation of a set of reference samples. The challenges in establishing objective metrological characteristics for visual test methods are associated with the significant contribution of the subjective factor-human eye color perception.

In terms of error, visual analysis methods are generally classified as semi-quantitative, though some publications report lower estimates of relative error for test methods. The metrological characteristics of a visual determination method are directly related to the color scale used. The analyte content in a control sample is assessed by applying the following rule: if the color of the control sample is intermediate between two reference points on the scale, the concentration is assigned the midpoint of the range. That is, if the unknown concentration of a component in the sample c falls between the n and n+1 members of the series, then c = (c_n + c_n+1) / 2. In this case, the relative determination error, termed the maximum error, is estimated as: s_r = Δc / c = (c_n+1 – c_n) / (c_n+1 + c_n).

If the color of the control sample matches a reference point on the scale, the formulas commonly used in instrumental analysis for calculating variance, standard deviation, and confidence intervals cannot be applied. In this case, methods of interval statistics should be used, where sample elements are not numbers but intervals. As the sample size increases (number of parallel determinations), the length of the intervals on the color scale does not change, meaning the parameter N only affects the precision of the mean estimate. The range of determination results will be limited either by the reference points on the scale or by the mean values of adjacent intervals on the scale.

Thus, despite the presence of subjective factors, the error in visual assessments does not exceed that of instrumental determination using a mini-photometer. The colorimetric analysis method allows for the creation of a uniform-contrast colorimetric scale that is easily distinguishable by the human eye.