Practical application of Sigma Metrics QC procedures in clinical chemistry.

BACKGROUND: Many labs have not yet selected the most appropriate Westgard Quality Control (QC) rule for each test. This is mainly due to the apparent complexity of the matter. METHODS: From the Westgard OPSpecs Charts QC planning tool and the Sigma Metrics formula's it was deduced that every Westgard rule has its own Sigma value. This was converted to an easy three-step road map to optimal Westgard QC rules. RESULTS: The road map provided is based on Sigma Metrics that hold a definition of "world class quality", at which no further effort to increase quality needs to be taken. Furthermore, it is shown that clinical chemical tests can be classified as "good": quality at or above world class, "bad": quality below world class but controllable with Westgard QC rules and "ugly": quality not controllable with Westgard QC rules alone. Finally, practical tips of how to deal with this and related aspects are given. CONCLUSIONS: The use of the road map based on Sigma Metrics leads to fast and easy implementation of optimal Westgard QC rules.

Reasons for ordering laboratory tests and relationship with frequency of abnormal results.

OBJECTIVE: Laboratory tests are ordered on a daily basis, even though disease probability is often very low. Abnormal results, especially mildly abnormal results, can be difficult to interpret in these circumstances. Further insights into the occurrence of abnormalities can help improve rational test ordering and test interpretation. The objective was therefore to examine the frequency of mildly and markedly abnormal results and their relationship with physicians' reasons for ordering tests. DESIGN: Prospective study. Participants. A total of 87 primary care physicians in the Netherlands collected data on 1775 patients. MAIN OUTCOME MEASURES: The physicians recorded the reason for ordering the tests, the most probable diagnosis and the pretest probability. The laboratories' reference values and specified "action limits" were used to assess the number of abnormal results and markedly abnormal results, respectively. RESULTS: Laboratory results were received for 1621 patients and 15,603 tests were reported (mean 9.6). The proportion of abnormal test results increased with increasing pretest probability (from 13.9% to 34.7%) and was 13.4% for tests ordered to reassure the patient and 13.3% for psychosocial diagnoses. The proportion of patients with at least one abnormal test result was high: 53.1% for tests ordered to reassure and 57.7% in patients with low pretest probability. Corresponding values for a marked abnormality were 11.1% and 12.4%, respectively. CONCLUSION: Abnormal laboratory test results were frequent, even when pretest probability was low. Physicians should therefore carefully consider when tests are necessary. Future research could explore physicians' interpretation of test results and its impact on diagnosis and management.

Correction of patient results for Beckman Coulter LX-20 assays affected by interference due to hemoglobin, bilirubin or lipids: a practical approach.

The influence of interference by hemolysis, icterus and lipemia on the results of routine chemistries may lead to wrong interpretations. On Synchron LX-20 instruments (Beckman Coulter) serum or plasma indices can be used as reliable semi-quantitative measures of the magnitude of such interference. In an article recently published in this journal, we presented the results of a multicenter study carried out in Dutch hospitals in which we determined cutoff indices for analytes above which analytically significant interference exists. Clinically significant interference cutoff indices were also derived for these analytes. In this article, we describe the handling of patient samples with clinically significant interference by hemolysis, icterus or lipemia. We investigated several possible approaches for correction of the result: dilution of the interference; mathematical correction in the case of hemolysis; treatment with ferrocyanide to destroy bilirubin; and removal of lipids in lipemic patient samples. We concluded, that mathematical correction of potassium or lactate dehydrogenase results in hemolytic samples can only be carried out if intravascular hemolysis is ruled out. Hemoglobin quantification in serial patient samples, combined with measurement of haptoglobin, represents a useful tool to rule out in vivo hemolysis. We derived an algorithm for this situation. We do not simply recommend mathematical correction, unless it is clinically acceptable. We present formulas for potassium and lactate dehydrogenase: corrected potassium=measured potassium-(hemolytic index increment x 0.14); corrected lactate dehydrogenase=measured lactate dehydrogenase-(hemolytic index increment x 75). The dilution studies indicated that dilution is only applicable for bilirubin, C-reactive protein and iron. The results of treatment with ferrocyanide were poor, and we do not recommend this method. Removal of lipids using high-speed centrifugation or LipoClear (StatSpin Inc.), a non-toxic and non-ionic polymer, is a very effective approach, although C-reactive protein, creatine kinase-MB (CK-MB) and cholesterol cannot be removed using LipoClear. For all interferants (hemoglobin, bilirubin, lipids), relatively simple algorithms are derived that can easily be implemented in the clinical laboratory.

Multicenter evaluation of the interference of hemoglobin, bilirubin and lipids on Synchron LX-20 assays.

The influence of interference by hemolysis, icterus and lipemia on the results of routine chemistries may lead to wrong interpretations. The H-, I- and L-indices that can be measured by the Beckman LX-20 instrument (Beckman Coulter) in serum or plasma samples are a reliable semi-quantitative measure of the size of these interferences. A survey carried out in 16 Dutch clinical laboratories on the use of these indices demonstrated that in several of these laboratories, the influence of interferences is largely underestimated. Therefore, a multicenter study was carried out in which we examined the interference of hemolysis, icterus and lipemia on 32 analytes. On the basis of biological variation, we decided on cutoff indices above which analytically significant interference exists. We found analytically significant interference by hemolysis, icterus or lipemia, in 12, 7 and 15 of the 32 analytes studied, respectively. Flagging of results on the basis of analytically significant interference, however, results in too many clinically insignificant comments. On the basis of clinical significance, we conclude that significant interference by hemolysis, icterus or lipemia is present in only 5, 6 and 12 of the analytes studied, respectively. Use of the cutoff indices presented here facilitates optimal use of the LX-20 indices to prevent reporting of wrong results due to interference.