A plethora of different molecular biomarkers’ screening tests are available for handling early-stage disease in an asymptomatic population. A basic classification divides them into immunoassays, targeting proteins (antigens, antibodies) and genomic assays of nucleic acids. To the best of our knowledge, a commercial platform that can handle, simultaneously, the ultra-sensitive assay of both protein and genomic markers, is not yet available. Screening tests are further classified into qualitative and quantitative tests. A qualitative test assays the smallest amount of a targeted marker that can be distinguished from the level of the noise (random error) at an acceptable level of confidence. This is addressed as limit-of-detection, LOD = noise-average-level (n) + 3 times the noise-standard-deviation (3·sigma), and results in false positives lower than 1% but false negatives up to 50%. Point-of-care, single use, lateral-flow, rapid screening tests generally work at the LOD. These are largely immuno-tests, but genomic markers can be targeted too, with time-to-results of tens of minutes. Drawbacks include large errors and LODs in the nM range, often too high to effectively serve in early diagnosis. Quantitative tests work beyond the limit-of-quantification (LOQ = n + 9·sigma) to quantify an analyte with an acceptable precision. i.e. both false positive and false negative lower than 1%, and gamma-error (overlap of gaussian error distributions of the LOQ and the LOD – type II error) lower than 1%. Yet, drawbacks are that the LODs do not always reach the single-molecule-level. Also, the need for a labelling step that contributes to the very long time-to results (several hours). Moreover, none can simultaneously handle assays for both protein and genomic markers. This is limiting the development of innovative and more efficient diagnostic protocols. This makes them inherently unsuited for detection at concentrations lower than pM. The LOD is the cut-off concentration below which the screening test becomes blind returning a negative (zero) response while yet, up to 100 million (ELISA) or thousands (Simoa) protein markers can be present in a 100 ul sample.
