A rapid semi-quantitative gradient lateral flow immunoassay (LFIA) of procalcitonin (PCT), a peptide precursor of the hormone calcitonin, was developed. The method is based on particular analyte cut-offs by immobilizing specific antibodies on the test strip with a consistent (gradient) increase in concentration from line to line. Semi-quantitative multi-range analysis is evaluated visually by counting the number of colored test lines corresponding to a certain concentration range of sepsis marker: [PCT]˂0.25; 0.25 ≤ [PCT] < 0.5; 0.5 ≤ [PCT] < 2; 2 ≤ [PCT] < 10; [PCT] ≥ 10 ng·mL-1. This multi-range gradient LFIA was implemented by using two types of label: spherical gold nanoparticles (35 nm) and hierarchical popcorn-like gold nanoparticles (100 nm). The comparison of this LFIA with an ELISA (for n = 82) yielded 87.5% and 76.6% sensitivities, and 92.3% and 92.3% specificities, respectively. Thus, multi-range gradient LFIA performs well at PCT thresholds, which is important for early diagnosis of sepsis and severe bacterial infection. In our perception, this method has a wide scope in that it may be implemented in numerous other LFIA based test systems. Graphical abstract Schematic of the gradient lateral flow immunoassay for determination of clinically relevant procalcitonin ranges. It allows to reach the correlation between the number of developed test lines and procalcitonin concentration range in serum by pre-immobilization of capture antibodies in a consistently (gradient) increasing concentration.
Keywords: Barcode; Cut-off adjustment; Diagnostic cut-offs; Gold nanopopcorns; Gold nanospheres; Immunochromatography; Sepsis marker; Test strip.