Delta-pH method for measuring blood cholinesterase. A study on the effect of temperature

Efforts to evaluate precision of the Michel delta pH method for blood cholinesterase, performed in a beaker immersed in a 25 C water bath, resulted in unexpected variability in results of daily and day-to-day replicate analyses. Most disturbing was the considerable variation between each four-hour period of an eight-hour day (AM and PM measurements). Precision of quadruplicate sets, expressed as coefficient of variation, ranged from plus or minus 0.5% to 8.5%. By rigidly controlling temperature with a water-jacketed reaction vessel the variation was reduced to plus or minus 1.7%. A study of effect of temperature on the method showed that a one-degree change in temperature resulted in a 5.5% change in plasma activity and a 3.0% change in red blood cell (RBC) activity. A graphing of the influence of temperature on reaction rate produced a linear relationship. Identical thermodynamic data were obtained with plasma and RBC samples that were inhibited 50% with an irreversible anticholinesterase agent.