Assay of erythromycin esters.

To the Editor: It was disturbing to read the article entitled “Studies of the Stability of 18 Chemical Constituents,” in the December, 1972, issue of Clinical Chemistry (1). The authors define their conditions of collection and storage but do not, however, make it clear what their object is. If the objective was to show stability of serum before analysis, then their data are misleading. For example in Table 1, the authors show a higher LD mean (329) for serum frozen than for sera kept at room temperature, 37#{176}C, or refrigerated temperatures. The reader might assume that serum should be frozen if the determination of LD is not to be carried out immediately. This assumption would be false since LD loses some of its activity (2) on initial freezing. Since the authors initially froze the serum before the start of their experiments, it should have been stated that any loss caused by initial freezing was not taken into account. The data as presented are excellent if the object was only to show stability of control serum collected and stored under the authors’ defined conditions.