One aspect of the present disclosure relates to a handheld device that can be used to perform a screening or diagnostic test. The handheld device can include a disposable microsampler unit, an analysis unit, a controller unit, and an output unit. The disposable microsampler unit can collect 10 microliters or less of a sample. The analysis unit can include two electrodes that can apply alternating periods of coulometry and potentiometry to the sample to determine a total content of a target chemical in the sample. During the coulometry period, the two electrodes are a working electrode and a counter electrode, and during the potentiometry period the two electrodes are an indicator electrode and a reference electrode. The controller unit can control the sequence of coulometry and potentiometry. The output unit can display the total content of the target chemical in the sample.
