Alkaline Phosphatase: Lability in Fresh and Frozen Human Serum and in Lyophilized Control Material

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Abstract
On storage, the activity of alkaline phosphatase (AP) in human serum increases. This has been known, but largely ignored, for 40 years. Improved instrumentation makes this lability more obvious and more important because errors from other sources have been decreased. With a stabilized spectrophotometer we quantitated the effects of time and temperature on AP activity of human sera and of lyophilized "control" material. Activities of 10 fresh sera increased by an average of 0.9, 2.7, and 6.1% in 6, 24, and 96 h; that of pooled serum, frozen and thawed, increased about 1% per hour. For 22 lots of reconstituted lyophilized material the increase ranged from 3 to 22% in 6 h. Refrigeration greatly decreased, but room temperature restored, the rates of change. Freezing decreased the activity of some lots, and most had a smaller rate of increase. Generally, the lower the initial activity of a given lot, the greater was the rate of increase. The difference in rate of increase among materials from different sources varied widely. Delay in measurement of serum AP activity is not the only source of potentially serious error; time and temperature variations are also, when lyophilized materials are used.

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