Abstract
All you ever wanted to know about the pregnancy test – and more! As the technology employed in the practice of laboratory medicine becomes more sophisticated, there is a risk that our understanding of the analytical science that underpins our specialty will diminish. Indeed, I suspect that this is already happening. If a test fails in the laboratory are we able to troubleshoot the cause effectively? Or do we call out the engineer, call the supplier's reagent specialist – or just get more reagent kits out of the refrigerator and try again? While these actions may be practised in the environment of a clinical laboratory, they are not really viable alternatives when the test is used in the home, the health centre, the local pharmacy or even the ambulatory clinic.
The core of this book comprises chapters that describe the evolution of the lateral flow immunoassay (LFIA) and then the various components of such a system. This is complemented by chapters on market trends and regulatory considerations. These two chapters are interesting as they describe the wide range of LFIAs available, and where they are primarily used. Thus, while pregnancy testing represents the major application for LFIAs (35% of the market in 2007), infectious disease and cardiac marker testing are catching up (20% and 21%, respectively). Interestingly, LFIAs represented 35% of the immunoassay market in 2007. I think these figures will be heavily influenced by practice in the USA, but it does show an interesting trend in testing outside the laboratory setting. The regulatory issues are again interesting, in that they indicate the information required to get a point-of-care testing device registered (approved for sale). This is effectively quality management in design and manufacture; important considerations for testing outside of a laboratory environment.
The first chapter discusses the key components of an LIA device including (i) the membrane or analytical reagent; (ii) the conjugate pad; (iii) the sample pad; (iv) the wick; (v) the backing material; and (vi) the choice of labels. The author discusses the purpose and construction of these components, illustrating how understanding of the processes and their chemistry have led to improvements, as well as the trends for the future. Subsequent chapters then deal with specific issues in more detail including choice of materials, antibodies, labels and membrane matrix, each giving a more detailed insight on how an LFIA is developed. Considering antibodies, for example, LFIAs require that these have very high affinities as the analyte (e.g. an antigen) may only be in contact with the antibody for between one and six seconds! In addition, the way in which antibodies are immobilized can have a major impact on performance.
The choice of labels for immunoassays has always been a key consideration and it is interesting to see their merits discussed in the context of LFIA development and performance. There is some interesting chemistry described from colloidal gold and colloidal carbon, through paramagnetic particles, fluorescent and luminescent labels to quantum dots and upconverting phosphors. Much of the rationale for the choice of labels will be lost on users today, but if you are involved in developing rapid and simple LFIA-type devices then the information is invaluable.
While some of the forgoing discussion may be relatively well known to the laboratory professional, uncovering the dark secrets of liquid flow, chemical interaction with surfaces, mixing and viscosity will be fascinating and can have a direct impact on practical issues like sample choice and suitability. The description of work undertaken to improve reaction matrices, the backbone of an LFIA device, indicates the underlying science that has been undertaken to develop these apparently simple to use testing devices.
While the early LFIAs were simple visually read devices, instrumentation has now been developed to enable quantitation of the LFIA response. Again, an understanding of the use of apparently simple readers will help in the choice of technology and troubleshooting problems that occur. However, this is where the device and reader combine together to extend the performance capabilities. Thus while the LFIA may be configured as a semi-quantitative device, e.g. for drug screening, the precision and accuracy of performance at the stated cut-off point has to be reliable. Developing a method to achieve this performance is as complex as developing a full quantitative method – and maybe we are too dismissive of semi-quantitative methods; from a clinical perspective they may perform extremely well in certain clinical settings.
This book gives a valuable insight into the development of the most commonly used ‘closed analytical system’. It contains useful information for any laboratory professional seeking to understand how modern day analytical methods work, and particularly for those involved with point-of-care testing. It is a required reading for anyone contemplating the development of an LFIA. However, beware: there is a minefield of intellectual property out there!