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The ImmunoCAP system (Thermo Fisher, Uppsala, Sweden) is a widely used ELISA technique for automated analysis of specific IgE antibodies. This system has also been found useful for automated analysis of IgG antibodies against a number of mold and bird antigens as part of the diagnosis of aspergillosis and extrinsic allergic alveolitis [1–3]. This method has now largely replaced the time-consuming and labor-intensive Ouchterlony double immunodiffusion technique, also known as the precipitin test [1, 2]. In Belgium and the Netherlands, an external quality control (QC) scheme, ‘Type III Allergy’, is operational within the section ‘Humoral Immunology’ of the SKML (www.skml.nl). In 2012, the results of four annual QC rounds were reported [4]. Here, we further analyze these results on the basis of paired QC samples. The outcomes suggest that such a paired analysis may uncover analytical bias in individual laboratories. Routine analysis of specific IgG antibodies was performed every week with the ImmunoCAP 250 system according to the manufacturer’s instructions (ImmunoCAP 250 User Manual, version 1.4, November 2012, Thermo Fisher), as described previously [4]. After routine analysis of specific IgG against Aspergillus fumigatus with the ImmunoCAP 250 system, sera were frozen at −20 °C. Every year, three new serum pools that contained a low, intermediate and high level of IgG antibodies against A. fumigatus were prepared for use in the external QC scheme [4]. These serum pools were composed of approximately 25 individual sera. NaN3 was added as a preservative at a final concentration of 0.01%. After aliquoting by SKML, the three QC samples were distributed together each year to a number of laboratories in Belgium and the Netherlands. The Institutional Review Board of Erasmus University Medical Center approves the use of leftover samples for QC purposes. GraphPad Prism for Windows, version 5.01 (GraphPad Software, San Diego, CA, USA) was used for statistical analysis of the results. In the external QC scheme, good agreement between the results from laboratories that used the ImmunoCAP system was previously reported [4]. In a further analysis of these QC data, the results for the intermediate and high IgG antibody QC samples in the individual laboratories were plotted against each other (Figure 1). A significant correlation was observed in all four annual QC rounds (2008: Spearman rank rs = 0.82, p < 0.0001; 2009: rs = 0.84, p < 0001; 2010: rs = 0.82, p < 0001; 2011: rs = 0.83, p < 0.0001). This result was unexpected: an independent distribution of the results for the intermediateand highIgG antibody samples was anticipated. Comparison of the results of the low IgG antibody QC samples with those for the intermediate IgG antibody QC samples showed a similar, although less significant, pattern (2008: rs = 0.41, p = 0.07; 2009: rs = 0.46, p = 0.04; 2010: rs = 0.80, p < 0.0001; 2011: rs = 0.70, p = 0.0008). The pattern, depicted in Figure 1, points to a constant ratio between the results of the paired QC samples analyzed in the different laboratories. This finding suggests that the paired results from the participating laboratories were read from standard curves with a similar shape, which nonetheless produced different quantitative results in the various laboratories. A possible explanation could be calibration curve drift. Phadia (now Thermo Fisher) has introduced a procedure for the ImmunoCAP system, in which the same aRetired *Corresponding author: Albert W. van Toorenenbergen, PhD, Department of Clinical Chemistry, Erasmus University Medical Center, Room NA-420, PO Box 2040, 3000 CA Rotterdam, The Netherlands, E-mail: toor.gen@gmail.com Joep J.M. Kurstjens: Department of Clinical Chemistry, Erasmus University Medical Center, Rotterdam, The Netherlands

Between-laboratory analysis of IgG antibodies against Aspergillus fumigatus in paired quality control samples