Coccidioidomycosis is a common infection within the area in which it is endemic, heralded by a spectrum of disease manifestations ranging from no symptoms (as seen in up to 60% of infected persons [9
]) to a respiratory infection of variable severity. It is typically characterized by one or more of the following symptoms: fever, chills, night sweats, headache, myalgia or arthralgia, cough, chest pain, shortness of breath, rash, and other manifestations (10
). Primary pulmonary coccidioidomycosis accounts for 15% to 29% of community-acquired pneumonia (1
) within the area of endemicity. When respiratory symptoms are present, many pulmonary coccidioidal infections are characterized by a dry, nonproductive cough. The dearth of easily expectorated sputum makes the diagnostic process more challenging, and serology is often used to secure the diagnosis. Serologic tests become positive at variable periods after the onset of illness (3
) and remain positive for months to years but typically resolve after resolution of illness (2
). Because patients may have no symptoms at the time of illness, the infection may be unrecognized clinically yet serologically positive. Therefore, the interpretation of a positive serologic test may be problematic.
A number of test methods are available to detect coccidioidal IgM and IgG. The most common serologic tests currently in use are CF, ID, and EIA. Tests to detect complement-fixing IgG antibodies are well established, are performed in reference laboratories, and have the advantage of being quantitative (2
). Optimal detection with CF peaks at approximately 1 to 2 months from the onset of symptomatic infection (3
). The CF serologic titers fall with progressive convalescence (2
). The detection of antibodies by ID is very specific and is often used to confirm a positive test by EIA; however, antibodies detected by ID may arise later in the course of infection than those detected by EIA (3
). Coccidioidal serology detected by ID is also determined in reference laboratories and may be reported as a quantitative result at certain laboratories (2
The EIA is a useful test because it can be done locally and has demonstrated better sensitivity in early coccidioidal illness than other methods (3
). Two EIA kits are available commercially, without discernible differences in performance between them; both are easy in use, have a rapid turnaround time, and can be performed in local laboratories. One retrospective study demonstrated that 86% of patients with this test result had positive serologic findings by other methods (5
), a finding echoed in the present study of 102 patients, in which 80 (78%) had either positive culture or histology or positive serologic findings by other methods. The demonstration of both IgM and IgG by EIA has been repeatedly shown to correlate well with other diagnostic assays in reference laboratories (6
), but the finding of the IgM+
remains problematic. Such problems with the IgM resolve when the serology is repeated (5
), and seroconversion from isolated EIA IgM reactivity to EIA IgG reactivity occurs. Such seroconversion was seen in 20% of patients in a prior study (5
). In the present study, seroconversion was observed in similar proportions of patients, whether initial serology had been performed for screening (5 of 29 patients [17%]; median seroconversion at 247 days) or for diagnostic purposes (7 of 60 patients [12%]; median seroconversion at 42 days). However, as illustrated by our findings, a minority of patients manifest such seroconversion, leaving health care practitioners with a diagnostic challenge in the case of IgM-only EIA reactivity.
The suboptimal clinical specificity of the EIA is explained partially by cross-reactivity with antigens to other fungi. Investigators have previously demonstrated problematic cross-reactivity with the EIA IgM serologic test in patients with known blastomycosis and other noncoccidioidal pulmonary illnesses (6
). However, within the area in which coccidioidal infection is endemic, it is uncertain that this cross-reactivity accounts for more than a small percentage of laboratory studies, since Blastomyces
organisms are not endemic in the same regions as Coccidioides
species. On the other hand, the area of the southwestern United States in which coccidioidal infection is endemic is a desirable geographic location that many people from distant locales either visit or relocate to, so cross-reactivity to other endemic fungi cannot be quickly dismissed.
A previous study conducted within the area of endemicity by Kuberski et al. (7
) demonstrated that a high percentage of isolated EIA IgM-reactive results were found in patients who were thought not to have coccidioidomycosis. Seventeen patients underwent testing by EIA for evaluation of symptoms and were found to have isolated EIA IgM reactivity. Samples were also sent to a reference laboratory for ID testing, and each patient's chart was retrospectively reviewed for an independent assessment of the presence of coccidioidomycosis (definitions were not provided). The isolated EIA IgM result was determined to be unconfirmed in 14 patients. The cohort consisted of 7 patients with clinical pneumonia, 5 with fever of unknown origin, and 4 with diagnoses not attributed to an infectious agent; the presence of compatible symptoms, radiographs, cultures, or serial serologic determinations was not presented for review. It is difficult to compare the results of the study by Kuberski et al. (7
) with the results of the present study, since the former study did not take into account culture or pathology results and used the reference laboratory serology as the “gold standard” by which a diagnosis was made (potentially problematic when the EIA may be seropositive earlier in the illness than ID studies [3
]). Furthermore, early treatment of an acute infection may have blunted an IgG response (14
). Finally, the former study had no evaluation for potential seroconversion.
In a comprehensive review of another center's experience with serologic diagnosis for coccidioidomycosis, Crum et al. (15
) found that 18% of isolated EIA IgM reactivity was unconfirmed. Similar to the findings of the present study, Crum et al. (15
) observed that the most unconfirmed results occurred in the year that more serologic tests were performed for surveillance purposes, rather than for the evaluation of symptoms. In the present study, 22 of 102 patients (22%) with isolated EIA IgM reactivity did not have a clear diagnosis of coccidioidomycosis that was confirmed by other serology or culture, and thus these may represent false reactivity. When we focused on patients who had serologic draws for screening purposes and not to evaluate symptoms, 16 of 29 patients (55%) had possible false-reactive results. An alternate explanation to a false-positive test result is that patients with coccidioidomycosis identified only by the newer EIA had fewer or milder clinical abnormalities than did patients in whom the infection was detected by older methods, as supported by a previous study (4
A recent study indicated that the IgG serologic response may be blunted or absent in patients with primary coccidioidomycosis who received antifungal treatment within 2 weeks of symptom onset (14
). Although 41 of 60 symptomatic patients (68%) in this study received antifungal medication, we did not find that the treated patients were less likely to experience a full serologic response than the untreated patients (P
= 0.07). However, information regarding the timing of such treatment during the course of illness was not collected for this study.
A strength of our study is that a large number of tests were performed and a single laboratory conducted the serologic testing. In addition, it is not uncommon in our practice to simultaneously perform EIA, ID, and CF, providing other serologic results to assist with test interpretation. A previous study showed that all 3 methods were necessary for the best interpretation because the methods performed differently with different patient populations (3
). Such ordering practices may not exist elsewhere on a routine basis. This study's origination from a tertiary referral center and retrospective design may also affect the generalizability of the results.
Although EIA is a sensitive and useful serologic test in early illness, the result of IgM-only EIA reactivity confers a diagnostic conundrum that may only be partially offset with culture, pathology, and repeated serology. The questions that remain present us with the ongoing need to better identify coccidioidal illness, especially early in the course of illness. A number of laboratories are evaluating novel assays to detect fungal antigens (rather than relying on a serologic result), and we welcome the opportunity to test such assays in the clinical arena. Until that time, we continue to study and better understand the tools that are currently available for clinical use.
In summary, the majority of patients in our study with isolated EIA IgM reactivity had coccidioidomycosis confirmable by other testing. Screening asymptomatic persons appeared to increase the rate of unconfirmable results but had a role in identifying active coccidioidal infections that required treatment during immunosuppression. Interestingly, seroconversion from IgM-only EIA reactivity was similar between the symptomatic and screening populations. Serial reassessment of coccidioidal serology may clarify a result of EIA IgM+/IgG− over time. Health care practitioners are advised to understand the advantages and disadvantages of the serologic methods used by their laboratory. We encourage future studies to identify and refine assays and methods that improve the ability to identify coccidioidomycosis in both symptomatic and asymptomatic patients.