Serum cholinesterase assay using a reagent-free micro pH-stat

Enzyme activities in body fluids are often used as diagnostic markers for physiological conditions and diseases. Common enzyme assays use optical methods that often require the use of pseudosubstrates and associated dyes. Here we introduce a reagent-free micro pH-stat that can determine absolute enzyme activity without the need for exogenic reagents. This approach employs electrolysis for precise dosing of the requisite acid or base titrant to stat the pH of the sample. The micro pH-stat is based on the rotating sample system (RSS), a convection platform for microliter drops. Activities of serum cholinesterase in fetal bovine serum and human serum were analyzed with this approach. The performance of this system is comparable to that of standard techniques (r2 = 0.99), yet it offers a broader range of detection. The reagent-free micro pH-stat has potential to be developed as a miniaturized device for point-of-care testing.