CV% for ESR.

CV% for ESR 

Q We are interested what is stated % CV for within-run imprecision for determination of erythrocyte sedimentation rate (ESR). Is there any published data? If there is not such, what value should we use for evaluation of a new analyzer? What CV values should I expect when running sed-rate daily controls?

A Sedimentation-rate QC can be run using commercially available control materials. Typically, this material has two levels: one in the low normal range, and one in the elevated range. The CV is influenced not only by environment and technique but also by the testing method used. CV is defined as the ratio of the standard deviation(s) to the mean ([micro]) and is often reported as a percentage, or CV = s/m. Therefore, the CV obtained when running sed-rate control is inversely proportional to the mean value for each level of control. Generally, the "normal" sed-rate control (level 1) produces a much higher CV than the "abnormal" sed-rate control (level 2), because the mean value of level 1 control is much lower than the mean value of level 2.

I was assisted in answering this question by Streck Inc., a manufacturer of a sedimentation-rate quality-control products. The company collects quality-control data from users of its product in a program called STATS.

Individual customer data from Streck's ESR control peer-group STATS database shows the range of mean, standard deviation, and CV values observed for three representative ESR methods as follows:

So far, we have discussed daily QC variability. The question asks, however, about within-run variation. Because some of the variables affecting day-to-day testing are less well-controlled, within-run variability is usually less than day-to-day variability.

Streck informed us, "To our knowledge, there are no published acceptance criteria for reproducibility in ESR testing. When qualifying a new ESR method for inclusion on the control assay, we conduct a reproducibility study using multiple lots of control material to compare the variability of the new method to the variability of a similar established ESR method. We have observed CV%'s for L2 (abnormal) control in the range of 2% to 10%. CVs for L1 (normal) control are substantially higher due to the smaller mean value of the L1 material. CV%s for fresh patient samples will vary according to the magnitude of the patient's ESR result. A patient specimen with an elevated sed-rate will produce a lower CV% than a normal patient specimen in a reproducibility study, just as L2 control has a lower CV% than L1 control."

[ILLUSTRATION OMITTED]

ESR Method            Level   Mean      SD         CV

Streck ESR-Auto Plus    1    6 to 8    1 to 2  14% to 28%
Diesse Mini-Ve          1    6 to 8    1 to 2  15% to 39%
Polymedco Sediplast     1    2 to 7    1 to 2  18% to 34%
Streck ESR-Auto Plus    2    79 to 93  2 to 6   3% to 7%
Diesse Mini-Ves         2    81 to 95  2 to 6   3% to 7%
Polymedco Sediplast     2    38 to 52  5 to 7  11% to 15%

--Daniel M. Baer, MD

Professor Emeritus 

Department of Pathology 

Oregon Health and Science University

Portland, OR

MLO's "Tips from the Clinical Experts" provides practical, up-to-date soluations to readers' technical and issues from a panel of experts in various fields. Readers may send questions to Dan Baer by e-mail at [e-mail].

Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.

Edited by Daniel M. Baer, MD