Dried Serum Spots for External Quality Assurance of Scrub Typhus IgM Serology: A Multicentre Pilot Study From India

Scrub typhus, caused by Orientia tsutsugamushi , is a significant contributor to acute undifferentiated febrile illness in South and Southeast Asia. Laboratory confirmation of infection has a crucial role in timely management; nevertheless, lack of standardisation in inter-laboratory performance of IgM enzyme-linked immunosorbent assay (ELISA) has been a long-standing challenge. In resource-constrained countries, quality assurance schemes have an important role in improving laboratory performance. The aim of this pilot study was to assess the applicability of dried serum spots (DSS) as a suitable sample format for quality assurance schemes of O. tsutsugamushi IgM ELISA in India.

Well-characterised positive and negative serum samples were processed into DSS panels and were systematically validated for homogeneity and short-term stability according to ISO/IEC 17043 and ISO 13528 using two commercially available IgM ELISA kits (InBios International Inc. and J. Mitra Scrub Typhus IgM Microlisa). Distribution was done for 10 participating laboratories across India. The laboratories tested DSS panels under routine diagnostic conditions using standardised protocols. The results were evaluated for qualitative accuracy, inter-laboratory concordance and overall performance.

DSS panels exhibited a high degree of homogeneity and stability under simulated transport and storage conditions, thereby not requiring cold chain distribution. On initial testing, 8 of 10 laboratories (80%) showed complete qualitative concordance across the five-sample panel. Following repeat testing and procedural review, all laboratories achieved concordant qualitative results, with minor disagreement noted only in borderline reactive samples.

Dried serum spots are a feasible, reliable and cost-effective alternative to conventional liquid serum for external quality assurance of O. tsutsugamushi IgM ELISA. The initial discordance observed underscores the importance of EQA in identifying procedural variability, while the final concordance confirms the robustness and suitability of DSS for EQA implementation in resource-limited, endemic settings like India.