Invasive and non-invasive measurement in medicine and biology: Calibration issues

Invasive and non-invasive measurement sensors and systems perform vital roles in medical care. Devices are based on various principles, including optics, photonics, and plasmonics, electro-analysis, magnetics, acoustics, bio-recognition, etc. Sensors are used for the direct insertion into the human body, for example to be in contact with blood, which constitutes Invasive Measurement. This approach is very challenging technically, as sensor performance (sensitivity, response time, linearity) can deteriorate due to interactions between the sensor materials and the biological environment, such as blood or interstitial fluid. Invasive techniques may also be potentially hazardous. Alternatively, sensors or devices may be positioned external to the body surface, for example to analyse respired breath, thereby allowing safer Non-Invasive Measurement. However, such methods, which are inherently less direct, often requiring more complex calibration algorithms, perhaps using chemometric principles. This paper considers and reviews the issue of calibration in both invasive and non-invasive biomedical measurement systems. Systems in current use usually rely upon periodic calibration checks being performed by clinical staff against a variety of laboratory instruments and QC samples. These procedures require careful planning and overall management if reliable data are to be assured.