Serum thiocyanate concentrations in patients with normal or impaired renal function receiving nitroprusside.

We have carefully reviewed the paper by De et al., reporting defective vials of NIST SRM 909a [1]. We thank the authors for initially alerting us to the problems they describe in some detail. While we do not disagree with their findings, we believe your readers should have additional information on SRM 909a. The Standard Reference Materials Program receives letters from SRM users from time to time and we take each one seriously as a source for investigation. De et al. point out a serious problem with the glucose concentration value in 909a. During the 14-year course of monitoring the stability of SRM 909, Human Serum, and its renewal, SRM 909a, we have found that the certified values for glucose decrease at an annual rate of-1%, whereas the concentrations of other analytes in these SRMs remain unchanged. As a result, all purchasers of this SRM have been periodically provided updated certificates with revised glucose values and uncertainties. Observations of degradation of measured glucose values in a lyophilized human serum matrix with time have also been documented in the literature [2]. After one of the authors contacted us about the glucose problem described in their paper, we investigated the remaining material in our inventory. We found that the beads of lyophilized serum in-10-15% of the vials had clumped together on the walls of the vials. Further inspection also revealed that some of the vials had improperly seated rubber septa and metal seals, which allowed moist air to seep in during storage. All vials with such clumping were removed from inventory and selected vials were analyzed. Later analyses confirmed that the glucose in the vials with the clumped material had decreased significantly (recoveries were <25%). Unfortunately, some customers had already received vials containing clumped material, and De et al. apparently were among this group of customers. Subsequent analyses of randomly selected vials from the remaining material without clumping found the glucose concentrations to be within the certified values and its uncertainties. It was further determined that the average moisture content (by mass) in the normal vials of SRM 909a serum was 0.4%, whereas the clumped material contained 10-12%. We applied the lessons learned concerning the moisture and storage problems with SRM 909a (no longer available) to the currently available SRM 909b. More-extensive monitoring and sampling protocols to detect substantive changes in the certified values of analytes such as glucose have now been …