We report a sensitive and versatile biosensing approach, LUCID (luminescence compact in-vitro diagnostics), for quantitative molecular and cellular analyses. LUCID uses upconversion nanoparticles (UCNPs) as luminescent reporters in mutually exclusive photo-excitation and read-out sequences implemented on a smartphone. The strategy significantly improves imaging signal-to-noise ratios, eliminating interference from excitation sources and minimizing autofluorescence, and thus enables filter-less imaging. Here we developed a miniaturized detection system and optimized UCNPs for biological applications. Nanoparticle luminescence lifetime was extended by controlling particles' structure and composition. When tested with a range of biological targets, LUCID achieved high detect sensitivity (0.5 pM for protein and 0.1 pM for nucleic acids), differentiated bacterial samples, and allowed profiling of cells. In proof-of-concept clinical use, LUICD demonstrated effective screening of cancer cells in cervical brushing specimens, identifying patients at high risk for malignancy. These results suggest that LUCID could serve as a broadly applicable and inexpensive diagnostic platform.