particles. This enabled the full recovery of MCV target values with both fresh and commercial blood preparations. The problem has recurred and been corrected twice by latex calibration of the red cell apertures.

At present the instrument is in control, according to the RB analysis, but the MCV results on our commercial controls are running consistently between 0 and 1 fem- tolitre above their assigned target values. We believe the 8 B analysis to be correct as this is based on years of laboratory experience, was confirmed with the loan instrument, and is correct according to current performances in both national and regional Ouality Assurance schemes.

To confirm our beliefs we calibrated the MCV with the commercial S Cal calibrant. This resulted in decreased MCV and increased MCHC values on fresh blood samples. The XB target value for MCHC rose from 33.6 g/dl to 34.5 g/dl, resulting in some patient MCHC parameters of greater than 3 5.4 g/dl. We regarded this as too high.

In conclusion, the discrepant MCV readings between fresh and stabilized blood samples, which have occurred over two years, appear to have been rectified by latex cali- bration. However, the MCV values of the commercial prep- arations still tend to be slightly higher than their assigned values.

I. Mellors C. Leyland

B. M. McArdle

Department of’ Huematology Freeman Hospitul Freeman Road High Heuton Newcastle upon l’yne, NE7 7DN

Corrcspondence to I . Mellors.

References

Bull B.S., ElashoKR.M., Heilbron D.C., Couperus J. (1975) A study of various estimators for derivation for the quality control pro- cedures from patients erythrocyte indices. Am. ]. Clin. Path.

Coulter W.H. (1956) High speed automatic blood cell count and cell size analyser. Paper presented at National Electronics con- ference, Chicago, IL, IJSA. October 3

Koepke J.A. & Protextor T.J. (1981) Quality assurance for multi channel haematology instruments, four years experience with patients mean erythrocyte indices. Am. J. Clin. Pathol. 75(1): 28

61(4), 473-481

Author’s reply

Sir: We appreciate the opportunity to respond to the

Q Bla~~!well Science Ltd, Clin. Lab. Haem. 1995, 17, 99-1 02

S-CAL* Calibrator issue included in the correspondence. Calibration of Erythrocyte Mean Cell Volume.

The observations reported by the Freeman Hospital con- firm routine accuracy assessments performed by Coulter Corporation. S-CAI, Calibrator assigned values are main- tained to within * 2% of whole blood reference assay. The difference reported in this publication is within the 2% limit for the S-CAL Calibrator assigned values.

The assay process for the assignment of values to S-CAL Calibrator incorporates adjustments to maintain accuracy within those limits. Feedback from our Interlaboratory Quality Assurance Program and other data supported a review of the assigned values.

Juan C. Covian

Director of Operutions Consurnabks Manufurtiiring Coulter Corporution Hiuleah E’loridu USA

Automated cell counters: quality control

Sir: It is evident that some laboratories use a weighted moving average of fresh patient samples (the XB algorithm, Bull & Elashoff 1974) to monitor the performance charac- teristics of automated cell counters. The circumstances in which this particular algorithm may be applicable were clearly laid down by its author. It is an absolute require- ment that each batch of 20 patient samples is a perfect reflection of the total patient population. To achieve this the samples must be randomized before analysis and within any batch of 20, no more than seven must arise from patients from one clinical source or having one clinical condition.

This algorithm was originally designed for a hospital in which all the samples were drawn very early in the morn- ing before the laboratory began work and in which all these conditions could be met. When the requirements cannot be met then the application of this algorithm will be invalid and any analysis based upon it will be misleading. IJnless a laboratory can show that these conditions can be fully met a false sense of security may result. Laboratories which receive and process a continuous stream of samples throughout the working day cannot apply the algorithm validly nor can those that receive and process batches of samples from one clinic or ward.

Any assertion that an automated cell counter is in con- trol based upon an invalid analysis can carry no weight. The widespread application of this algorithm is largely the

Correspondence

result of its inclusion in software packages provided with automated cell counters. This has tempted some lab- oratories to adopt it even when they cannot meet the obligatory randomization conditions. It would be a great service to Haematology if equipment manufacturers who have made the XB algorithm available would spell out clearly the conditions under which the algorithm may be applied.

I. Cavil1

llepurtment of Huema tology University of Wales College of Medicine CurdijJ

References

Bull B. & Elashoff R.M. (1974). The use of patient derived hae- matology data in Quality Control. Proceedings oJ the Sun Diego Biomedical Symposium 13, 5 15-519.

0 Blackwell Science Ltd, CIirr. Lab. H a m . 1995. 17, 99-102