**2. "Big Five Demands" for new solutions in LFIA**

However, the application of LFIA did not stop at the control of the formed row of objects. This method actively developed (especially in the last decade) and covered an increasing number of analytes. What were the reasons for this?-


Taking into account this expansion of controlled analytes and types of tested samples, tasks were frequently encountered for which highly sensitive detection was required but not provided by traditional analytic formats.-

During the last decade, the development of LFIA modifications has been intensified, allowing highly sensitive analysis, while maintaining the basic merits of the analysis—the rapidity, ease of implementation and interpretation of the results. These developments are systematized in a number of recent reviews that characterize the general trends in the development of LFIA, its application in different practical spheres, and the most successful methodological decisions [2–8].-

On the one hand, this progress is accompanied by the expansion of the assortment of commercial tests and the more active application of LFIA for solving a variety of practical problems. On the other hand, a significant part of new developments remains at the level of single publications and approbation using the example of a single analyte, without realistic assessment of their advantages and limitations. From such isolated examples, it remains unclear how much gain in sensitivity will be achieved if we apply the proposed approach to the new analytes and what conditions must be used for this. A simple demonstration of the minimum detectable concentrations in traditional and modified LFIA leaves open the question of how correctly all the conditions for the analyses were selected, including the concentration and composition of the immunoreagents. It is also unclear which of the approaches for reducing sensitivity can be combined and whether this combination leads to a multiplication of results improvements achieved for each of these approaches individually.-

Of course, general theoretical arguments are not enough to answer these questions. Further studies of many research teams are needed. However, it is important to evaluate new developments with the use of a grounded concept to understand (i) what changes are introduced into the traditional LFIA protocol and for what purpose; (ii) by what criteria are the new LFIA protocols assessed and compared with existing ones. Such ordering is the subject of this review. We did not attempt to form a limited list of developments that are most widely represented in recent publications. Our goal was to create a general classification within which different existing and future developments can be characterized.-

The structure of the immunochromatographic test system considered (**Figure 1**) allows us to identify groups of problems that should be solved to ensure high-sensitivity, as well as other practically significant characteristics of the analysis (productivity, selectivity, etc.).

**Figure 2.** Compounds of immunochromatographic test and "big five demands" associated with them.-

We matched each element of the test system and the reagent or process used at this element. Therefore, the choice of the most appropriate (proper) actions during the analysis includes-


These five groups of requirements ("big five demands", **Figure 2**) make it possible to simply and- uniquely classify the methodical solutions proposed for the improvement of the LFIA protocols.-

This review will be based on our results (from the Laboratory of Immunobiochemistry in the A.N.-Bach Institute of Biochemistry of the Federal Centre of Biotechnology of the Russian Academy of Sciences, Moscow, Russia) and on examples from the literature that will be ordered and characterized in accordance with this classification.-
