Table 27. External comparison of IQCs.

Recommended practice	Evaluation of the truthfulness of the peer group average. Monitoring of CV (indicator) and SDI ratios on a regular basis (monthly or batch): a CVI greater than 1 reflects a performance that needs to be monitored (28). Use SDI values (Z-score) greater than 2 as an alarm. A Z-score higher than 3 indicates a difference with the other participants (caution: the Z-score depends on the dispersion of the results, to be interpreted according to the population of participants).
Acceptable practice	Temporary use of peer average as internal laboratory average. Temporary use of peer standard deviation to calculate acceptable limits for laboratories.
Unacceptable practice	Use of IQCs with peer mean and standard. Incorrect interpretation of data (SDI and CVI). Absence of reaction in the event of lasting performance degradation, without justification.
CV – coefficient of variation. SDI – standard deviation. IQC – internal quality control. CVI – intra-individual biological variation.