Cardiac troponin I (cTnI) has been used clinically, for years, for diagnosis and risk stratification in patients with suspected acute myocardial infarction (AMI)¹. Generally, elevated levels of cTns are conjoined with damage of cardiac muscles and cTnI release to the circulation has specificity for cardiac injury². Measurement of circulating levels cTnI, particularly observing the rise or fall in cTnI levels, along with the evaluation of patient symptoms, history and electrocardiographic (ECG) abnormalities are current procedures in the triage of suspected AMI patients^3,4. The typical challenge is early identification of AMI among the large heterogenous patient population, as only one in three patients, experiencing chest pain, who visit emergency department (ED) receives AMI diagnosis⁵. Therefore, timely and correct diagnosis to rule-in AMI is important in terms of cost savings and improving the clinical outcomes of AMI patients.

The use of UCNP measurement technology can enhance the assay sensitivity in comparison to traditional down-converting fluorescent reporters since it enables elimination of the background autofluorescence from the measurement. This is due to the unique photon upconversion luminescence of UCNP reporters converting low-energy excitation (near or at infrared) wavelength into high-energy emission at visible wavelengths¹¹. The UCNP signal can be read with a miniaturized relatively low-cost reader instruments with high performance in POCT settings. Such readers with miniaturized optics for UCNP measurement have previously been described in the literature^12,13,14,15.

The performance of LFIAs may suffer from interferences originating from the sample matrix, particularly because of the typical one or two step assay procedure with limited washing. These immunoassay (IA) interferences may be caused by different interfering agents such as circulating heterophilic IgG and IgM antibodies¹⁶, autoantibodies targeted against the biomarker of interest¹⁷ and human anti-animal antibodies¹⁸. In general, IgG antibodies used as binders in IAs are prone to several interfering reactions caused by circulating antibodies.

If a heterophilic antibody has high affinity for the Fc-region of an IA binder antibodies, an aggregate of binder antibodies with large numbers of Fc-regions close together functions as a binding target for the heterophilic antibody¹⁶. Furthermore, it has been shown that complement activation by the classical pathway can occur when several IgG molecules are in close proximity, e.g. on reporter nanoparticle surface¹⁹ or on the solid phase²⁰. As a results, complement factors bind to the Fc region of the IA binder antibody^20,21 which causes negative interference in the IA. Non-specific binding of autoantibodies such as rheumatoid factor (RF) has been associated with falsely elevated analyte concentrations²². Since IAs utilizing nanoparticle reporters aim at extremely high sensitivities, the effect of matrix interferences should be addressed to fully utilize the potential of nanoparticle IA technologies.

It has been depicted that the effects of interfering agents can be eliminated with sample pre-treatment procedures such as heating or precipitation of interfering antibodies with polyethylene glycol (PEG)²⁰. In addition, pretreatment with Ethylenediamine tetra-acetic acid (EDTA) effectively inhibits the activation of the complement pathway²³. However, particularly in the case of POCT, time-consuming sample pre-treatment steps are undesirable. Therefore, in LFIAs the potential solutions should be implemented in the test device itself.

In this study, we report the development of a UCNP-LFIA for cTnI. UCNP reporter technology was utilized to provide the sensitive and quantitative detection of cTnI in LFIA format to overcome the typical limitations of LFIAs. The matrix interference in plasma samples was studied and reduced by incorporating the developed sample pre-treatment steps into the LF strip. The performance of the developed UCNP-LFIA was evaluated with clinical plasma samples (n = 262).