1. SUMMARY VALIDATION of the COMPLEMENT COMPONENT C1q RID ASSAY Igor Y. Pavlov, Julio C. Delgado ARUP Institute for Clinical and Experimental Pathology, Department of Pathology, University of Utah School of Medicine, Salt Lake City, Utah INTRODUCTION MATERIALS and METHODS RESULTS CONCLUSIONS REFERENCES Radial immunodiffusion assay from (The Binding Site Limited, Birmingham, U.K) was applied to 81 healthy donor plasmas for reference interval determination, 29 serum/plasma pairs of the healthy donor specimens, and to samples with low, medium and high C1q concentrations to access accuracy, linearity and reproducibility of the assay. EP Evaluator software was applied to the result interpretation. Eighteen EDTA plasma samples were sent to the Quest Diagnostics, National Jewish Health Center (NJHC), and run at ARUP for C1q assays. Both Quest and NJHC are utilizing home-brewed radial immunodiffusion kits. During the validation study we found that C1q RID assay was accurate and linear in the range between 30 and 230 µg/mL with allowable error of 20% with 50% systematic error. Recovery of the spiked samples varied between 99% and 110%. Reproducibility from run to run and within runs was within 10%. By the nature of radial immunodiffusion assay, detection of C1q concentration below 50µg/mL (corresponding to the ring diameter below 5.0 mm) is not reliable: potential reading error of 0.1 – 0.2 mm generates 15% – 30% error in C1q concentration. Reference interval calculated with transformed parametric was estimated to be from 109 µg/mL (90% confidence limits from104 to117 µg/mL) to 242 µg/mL (90% confidence limits from 227 to 258 µg/mL). Comparing results of C1q determination by ARUP, Quest and NJHC, we found no correlations. Obviously, all 3 kits utilize different proteins as a standard. When results were normalized by the average value of each laboratory set, normalized data sets correlated to each other: confidence intervals for slopes included 1, and confidence intervals for the intercept included 0. Correlation Coefficient R between ARUP and Quest results was very poor, only 0.4; NJHC vs ARUP: R=0.87. Bias for ARUP – Quest comparison was twice as high (30% vs 15%) and confidence intervals for slope and intercept were much wider than for the NJHC – ARUP comparison results. Complement component C1q radial immunodiffusion assay was validated and ARUP reference interval defined. Direct comparison to other laboratories results should not be considered because of differences in utilized standards and, accordingly, reference intervals. C1q is a 400kDa hexameric gamma-2 protein that is a subunit of C1 – the first component of complement system. The binding of two or more of C1q’s six globular domains initiates the classical pathway of complement activation. C1q binds readily to the CH2 domains of aggregated IgG molecules in an immune complex or the CH3 domain of a single IgM molecule whose conformation has been altered following antigen binding. It can also bind directly to certain microorganisms and mycoplasmas. The multivalent binding of C1q is believed to lead to a conformational change in C1q complex, activating C1r, then C1s and thereby initiating the classical complement pathway. The collagen-like tail domain of C1q (which is only exposed once C1 inactivator dissociates C1r2 and C1s2 from the C1q-activator complex) increases the phagocytosis of particles by monocytes and macrophages. Serum levels of C1q are reduced in immune complex disease, SLE and meningitis. Hereditary deficiency is also known 1. Cooper NR (1985). The classical complement pathway: Activation and regulation of the first complement component. Adv. Immunol. 37, 151-216 2. Arlaud, GJ et al (1987). A functional model of the human C1 complex. Immunol. Today, 8, 106-111. 3. Ross, SC & Densen, P. (1984). Complement deficiency states and infection: Epidemiology, pathogenesis and consequences of neisserial and other infections in an immune deficiency. Medicine, 63, 243-273 4. Schifferli, GA et al (1986). The role of complement and its receptor in the elimination of immune complexes. New Eng. J. 315, 488-495 C1q is a subunit of the C1 complex that activates the classical pathway of the complement system. The Binding Site radial immunodiffusion assay was validated for the human EDTA plasma samples. It was found that C1q RID assay was accurate and linear in the range between 30 and 230 µg/mL. Recovery of the spiked samples varied between 99% and 110%. Reproducibility from run to run and within runs was below 10%. Reference interval was estimated to be from 109 µg/mL to 242 µg/mL. Comparing results of C1q determination by ARUP, Quest and National Jewish Health Center (NJHC), we found no correlations. Obviously, all 3 kits utilize different proteins as a standard. When results were normalized by the average value of each laboratory set, normalized data sets correlated to each other. Correlation Coefficient R between ARUP and Quest results was very poor, only 0.4; NJHC vs ARUP: R=0.87. Bias for ARUP – Quest comparison was twice as high (30% vs 15%) and confidence intervals for slope and intercept were much wider than for the NJHC – ARUP comparison results. Direct results comparison between different laboratories should not be considered because of the differences in utilized standards and, accordingly, reference intervals. Figure 1. Accuracy and linearity of the C1q RID assay Figure 2. Normalized data: ARUP vs National Jewish Health Center comparison Figure 3. Normalized data: ARUP vs QUEST comparison