TABLE 2. Reasons for failure to meet confirmation criteria* among probable† spotted fever rickettsiosis cases (N = 16,640) — United States, 2010–2015.
| Reason | No. (%) |
|---|---|
| Paired IFA IgG testing performed within recommended date range, without evidence of seroconversion |
218 (1.3) |
| Paired IFA IgG testing performed outside of recommended date range |
1,268 (7.6) |
| Supportive evidence demonstrated with IFA IgM, ELISA, dot-ELISA, or latex agglutination only |
1,597 (9.6) |
| Single positive IFA IgG titer§ | 13,557 (81.5) |
Abbreviations: ELISA = enzyme-linked immunosorbent assay; IFA = indirect immunofluorescence assay; IgG = immunoglobulin G; IgM = immunoglobulin M; IQR = interquartile range.
* Laboratory confirmed criteria: serological evidence of a fourfold change in IgG-specific antibody titer reactive with spotted fever group antigen by indirect IFA between paired serum specimens (one taken in the first week of illness and a second 2–4 weeks later), or by polymerase chain reaction, immunohistochemistry, or cell culture. A confirmed SFR case is clinically compatible (meets clinical evidence criteria: any reported fever and one or more of the following: rash, eschar, headache, myalgia, anemia, thrombocytopenia, or any hepatic transaminase elevation) and is laboratory-confirmed.
† Laboratory-supportive criteria: serologic evidence of elevated IgG or immunoglobulin M antibody reactive with spotted fever group antigen by IFA, enzyme-linked immunosorbent assay (ELISA), dot-ELISA, or latex agglutination. A probable SFR case is clinically compatible and has supportive laboratory results.
§ IFA IgG titer results are considered positive if ≥1:64.