Oximetry module for blood gas analysis: performance assessment under misalignment conditions.

The development of medical devices for diagnostics is one of the greatest challenges in the biomedical industry. A significant revolution in diagnostics occurred with the development of medical devices designed to be used outside the clinical laboratory and in close proximity to where the patient is receiving care. These instruments belong to the family of Point of Care systems and play a key role in many circumstances being crucial in clinical care settings and emergencies. This thesis will focus on the cobas b 123 Point of Care system, a cartridge based mobile blood gas and electrolytes analyser designed for critical point of care testing. In particular, this study focuses on the oximetry module installed in the instrument, an optical unit that allows the measurement of the concentration of hemoglobin and its derivatives within a blood sample. The oximetry module consists of various electronic, mechanical and optical elements that enable a light signal to be translated into a concentration value by exploiting the interaction between blood and light. The module is assembled following a specific process to ensure its perfect functioning and then its optical components are subjected to an alignment process whose purpose is to ensure that the signal is obtained and transduced in the most effective way through the light path. After the alignment procedure, before inserting the module into the instrument, its performance is evaluated by measuring first quality control material and then bovine blood. According to the test documentation, an abnormal trend towards test failure was observed over a certain period. More precisely, the modules in question passed the test when analysing quality control material, but failed the test with bovine blood resulting in hemoglobin values outside the acceptable range with an abnormal distribution. The aim of this thesis is therefore to understand the role that the alignment process has on the module performance by investigating possible relationships between misalignment and measurement results. Furthermore, it is intended to understand the reason for the failure of the tests with bovine blood despite the positive results of the tests with the control material. In order to investigate potential dependencies, the measured parameters and their clinical significance are initially studied in detail, as well as the theoretical method on which the operation of this optical module is based. Then the individual components of the module and their role are analysed from a functional point of view to identify any potential criticalities. The diffraction grating contained in the optical module resulted to be one of the critical elements during the alignment, therefore it has been decided to build a test set up able to reproduce various alignment conditions. The setup is then used to perform measurements with control material first and with blood later under different conditions of misalignment in order to identify a relationship between the results. The first step consists of identifying a misalignment range that allows positive results to be obtained using the control material, then under the same conditions, several blood samples are analysed and the results are evaluated by comparing different types of samples. Finally, it is assessed whether a misalignment condition undetectable by testing quality control material could lead to erroneous results with blood samples.