We describe a quadruple-label fluorometric immunoassay for simultaneously measuring four analytes: thyroid-stimulating hormone (TSH), 17 alpha-hydroxyprogesterone (17 alpha-OHP), immunoreactive trypsin (IRT), and creatine kinase MM (CK-MM). The assay is based on immunoreagents labeled with four different lanthanide ions (Eu3+, Tb3+, Sm3+, and Dy3+), on dissociative fluorescence enhancement applying the principle of co-fluorescence, and on time-resolved fluorometry. The monoclonal anti-alpha-TSH and anti-IRT antibodies and the polyclonal anti-CK-MM antibody were labeled with Eu3+, Sm3+, and Dy3+, respectively; 17 alpha-OHP was labeled with Tb3+. The assay was performed in microtitration strip wells coated with a mixture of monoclonal antibodies against beta-TSH, IRT, and CK-MM and a polyclonal goat anti-rabbit IgG for capture of the rabbit anti-17 alpha-OHP antibodies. After completion of the immunoreactions, the bound fractions of the lanthanides were dissociated into the co-fluorescence enhancement solution, creating highly fluorescent chelates. The four lanthanide-specific signals were subsequently measured in a time-resolved fluorometer. The detection limits of the assay were 0.1 mIU/L for TSH, 2 nmol/L for 17 alpha-OHP, 2 micrograms/L for IRT, and 4 U/L for CK-MM.
The availability of an intrinsically fluorescent, inert, and stable Eu chelate label made it feasible to design one-step all-in-one immunoassays with time-resolved fluorometry for detection. Both competitive and noncompetitive immunoassays are performed in microtitration wells containing all assay-specific components in a stable dry form. Only the sample and one assay buffer common for all analytes need to be added. Model assays for human chorionic gonadotropin (hCG), alpha-fetoprotein (AFP), and progesterone all reached equilibrium in 15 min or less without compromising the performance characteristics of the measurements, all of which perform at least equivalent to state-of-the-art assays. The detection limits for hCG, AFP, and progesterone were 0.3 IU/L, 0.1 microgram/L, and 0.5 nmol/L, respectively. The assay ranges for hCG and AFP were linear to 5000 IU/L and 1200 micrograms/L, respectively. The immunoassay format can be readily implemented in a fully automated random-access immunoassay system with optimal performance characteristics and no handling of analyte-specific assay components.
Background: Fluorescence resonance energy transfer (FRET) is a powerful tool in analytical chemistry. The aim of the present work was to use FRET to design a homogeneous immunoassay.
Methods: We used a highly fluorescent terbium (Tb3+) chelate (donor) and the organic fluorochrome rhodamine (acceptor) combined with time-resolved detection of the acceptor emission in homogeneous assay format for the measurement of the β subunit of human chorionic gonadotropin (βhCG) in serum. We used two antibodies labeled with Tb3+ and rhodamine, respectively, recognizing different epitopes on βhCG. The close proximity between the labels in the immunocomplex permitted energy transfer between the pulse-excited Tb3+ donor (decay time >1 ms) and the acceptor rhodamine (decay time of 3.0 ns). The prolonged emission of donor-excited acceptor (energy transfer) was measured after the short-lived background and acceptor emissions had decayed. The emission of donor-excited rhodamine was measured at a wavelength of where the emission of unbound donor is minimal.
Results: The energy transfer signal was directly proportional to the βhCG concentration in the sample. The limit of detection was 0.43 μg/L, and the assay was linear up to 200 μg/L. Total assay imprecision in the range 10–185 μg/L was between 7.5% and 2.8%.
Conclusions: Although less sensitive than heterogeneous, dissociation-enhanced europium-based separation assays, the presented assay format has advantages such as speed and simplicity, which make the assay format ideal for assays requiring a high throughput.
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