An in vitro method of determining an analyte concentration of a sample includes placing the sample into a low-field, bench-top time-domain nuclear magnetic resonance (TD-NMR) spectrometer. The NMR spectrometer is tuned to measure a selected type of atom. A magnetic field is applied to the sample using a fixed, permanent magnet. At least one 90 degree radio-frequency pulse is applied to the sample. The radio-frequency pulse is generally perpendicular to the magnetic field. The 90 degree radio-frequency pulse is removed from the sample so as to produce a decaying NMR signal. The decaying NMR signal is measured at a plurality of times while applying a plurality of 180 degree refocusing radio-frequency pulses to the sample. The analyte concentration is calculated from the plurality of measurements associated with the decaying NMR signal and a selected model.