To determine the concentration of a certain analyte in a sample, we need to measure some physical magnitude, some signal, that depends on this concentration and ideally only on it. That is, we need a measurement procedure that is sensitive and specific to the concentration of the analyte we want to find out. For this purpose, biochemistry systems use reagents specifically designed for each analyte. By mixing these reagents with a small volume of the sample, certain biochemical reactions are triggered that modify the optical properties of the mixture, depending on the concentration of the analyte we wish to measure. These reactions can cause the appearance or disappearance of certain substances, chromophores, that absorb light of a certain wavelength. In other cases, the reactions can lead to the formation of aggregates that scatter light, i.e., produce turbidity in the mixture. By measuring these changes in the optical properties of the mixture and comparing them with the changes observed with samples of known concentration, the analyser can determine the concentration of the studied analyte.

The complete measurement process includes the addition of reagents and samples, their agitation, their thermal control, the measurement of the mixture's absorbance, the calculation of the analyte concentration, and the visualization of the obtained results. Analysers fully automate this process and can perform it continuously and coordinatedly for many different analytes and patients during the execution of a work list.

The BioSystems BA400 analyser can start preparing a new reaction every 9 seconds. Its pipetting system, with separate ceramic piston pumps for reagents and samples, can dispense volumes of a few microliters with a precision greater than 0.5%, which directly impacts the good precision of the reported concentration results. For reagents preservation, the rotor containing them is refrigerated, and the pipetting tips are thermostated to raise their temperature at the time of dispensing them in the reaction rotor. The rotors have integrated barcode readers to securely identify both the reagents and the patient samples. The 120 cuvettes of the reaction rotor are thermostated at 37°C with a Peltier cell system that ensures high temperature stability of the reactions. Once the reagent and sample are dispensed into one of the cuvettes, the analyser agitates and homogenizes the mixture using a paddle stirrer. Subsequently, if necessary, the analyser can dispense a second reagent into the cuvette, using a dedicated tip and pump, and agitate the complete mixture using a second stirrer.

The optical system of the analyser emits light that passes through the reaction cuvette and is captured by a detector. As the reaction develops, substances that absorb or scatter light of certain wavelengths appear or disappear, altering the light intensity received by the detector. The physical magnitude that quantitatively measures the absorption or scattering of light is absorbance, which the analyser determines by comparing the amount of light measured during the reaction with the amount of light measured when there is only water in the reaction cuvette. Lambert-Beer's law indicates that absorbance is proportional to the concentration of substances that absorb light at a certain wavelength.

The BioSystems BA400 analyser can have up to 70 different reactions in progress at the same time and must measure the evolution of the absorbance of all of them for more than 10 minutes. To do this, its optical system with LED illumination can perform 24 optical readings at different wavelengths per second. This system, besides being very precise in its measurements, is extremely robust and maintenance-free as it has no moving mechanical parts and its light sources are LEDs. In one hour of work, it can perform more than 27,000 absorbance measurements with minimal energy consumption.

From the absorbance measurements of the reaction, the analyser calculates the desired analyte concentration. To do this, depending on the type of reaction produced by mixing the reagent with the sample, it selects certain absorbance values and calculates specific parameters following the protocol determined by the appropriate analysis mode for that reaction. For example, for an endpoint mode reaction, the analyser simply considers the absorbance value at a certain time. In contrast, for a kinetic mode reaction, the analyser calculates the rate of change of absorbance over a defined period of time. Finally, the analyser obtains the desired concentration by comparing these parameters with those obtained during the system's calibration with one or more samples of known concentration. The BioSystems BA400 analyser can report a new concentration result every 9 seconds. The results are displayed on the user program of the analyser and can be directly exported to the laboratory information management system (LIS) or printed in a report for each patient.