Quantitative, False Positive, and False Negative Issues for Lateral Flow Immunoassays as Exemplified by Onsite Drug Screens

Lateral flow immunoassay devices offer many advantages including convenience, economical, simplicity, and rapid result. Many lateral flow immunoassays are non-instrumental and rely on visual detection of colored lines for results, enabling easy portability and allowing testing at any time and at any place by non-technical personnel. Hence, many lateral flow immunoassay tests have been developed for use ‘‘onsite’’, ‘‘point-of-care’’, or ‘‘point-of-test’’. However, utilization of this type of device requires the acceptance of some trade-offs. The most important is that the test results are generally qualitative. Moreover, since it is antibody-based, possibility exists that chemicals with similar structures will cause positive result leading to specificity and sensitivity issues. Recognizing such limitations, this technology has been widely utilized for screening tests in which a yes and no answer is sufficient. The initial screen test result can then be confirmed by a quantitative method, which is usually equipment-based and required the service of highly trained technician. Acceptance of this two-step procedure allows the massive testing of subjects economically and quickly without worrying about the test locations and the service of highly trained technician. An example of one such application is at home pregnancy testing in which a positive result would inevitably lead to a doctor’s visit for further testing. Another example is workplace drug testing in which positive result from a drug screen is confirmed by a laboratory GC/MS or LC/MS result. With the increasing popularity of lateral flow immunoassay devices, many users have ignored the inherent qualitative nature and specificity/ sensitivity issues of the assays and placed increasing demands on the manufacturers for fool-proof onsite tests. The present chapter examines this issue as exemplified by the false results encountered in abused drug screens. We shall first examine the definitions of positive and negative results in a