Principle of Reflectance Photometry

Reflectance photometry quantifies the intensity of the colored product produced on the reagent strip reaction pads. When light strikes a matte or unpolished surface (e.g., a reagent strip), some light is absorbed and the remaining light is scattered or reflected in all directions. The scattered light is known as diffuse reflectance. In reflectance photometers, the incident light is usually of one or more wavelengths, whereas only reflected light of a single, specific wavelength is detected. These photometers are calibrated using reflectance standards such as magnesium carbonate or barium sulfate that “completely” reflect all incident light. Because the potential colors that develop on each reaction pad dictate the wavelengths of light needed for reflectance measurements, each reflectance photometer must have a way of selecting the appropriate wavelength for each test pad. To obtain the desired wavelength, reflectance photometers use (1) polychromatic light and a series of filters to isolate specific wavelengths or (2) a series of monochromatic light sources (e.g., light-emitting diodes [LEDs]).

Reflectance measurements are performed at specific wavelengths and are expressed as percent reflectance (% R). The % R is the ratio of the test pad reflectance (R_t) compared to the calibration reflectance (R_c), multiplied by the percent reflectivity of the calibration reference, which is usually 100%.

% R=RtRc×100 Equation 16.1

Equation 16.1

The relationship between concentration and reflectance is not linear. Therefore a microprocessor is needed to apply complex algorithms that convert the relationship to a linear one and to obtain a semiquantitative analyte value for each reaction pad on the test strip.

Semiautomated Chemistry Analyzers

The term semiautomated urinalysis indicates that an analyzer is used to interpret the commercial reagent strip results of urine when the chemical examination is performed. The term semiautomated indicates that the user performs the remaining steps of the urinalysis—physical examination of color and clarity, as well as the microscopic examination, if performed.

Fully Automated Chemistry Analyzers

When using a fully automated urine chemistry analyzer, the user simply places labeled tubes of urine into a sample rack or carousel. Testing is initiated by pressing a button on the instrument display or automatically with placement of a sample rack. From this point on, the instrument controls movement of the specimen rack, identifies each sample, mixes it, aspirates urine using a sample probe, and dispenses it onto a reagent strip. At the appropriate read time, each reaction is read using the appropriate wavelengths of light for that specific test.

Fully automated urine chemistry analyzers can also determine the physical characteristics of urine—color, clarity, and specific gravity (SG)—however, the methods used vary. To perform urine color assessment, some manufacturers include an additional pad on the reagent strip to determine urine color by reflectance photometry; others use spectrophotometry at multiple wavelengths to assign color. Light transmittance or light scatter is used to determine urine clarity. Despite the universal availability of an SG reagent strip test, most fully automated chemistry analyzers use refractive index because of its greater accuracy. A microprocessor collates all results—color, clarity, SG, and chemistry tests—which are subsequently sent to data storage and can be printed on a report form or sent electronically to an LIS.

Digital Flow Morphology

The iQ200 Microscopy Analyzer can be purchased as a standalone instrument. It is an automated system that performs the microscopic examination of urine, as well as cell counts on body fluids (see Automation of Body Fluid Analysis section, later in this chapter).