Many lymphatic filariasis patients are amicrofilaraemic, and because no serologic test other than that detecting CFA is specific, in the absence of antigen testing the diagnoses of these infections must be made 'clinically' (i.e., on circumstantial evidence) with support from antibody or other laboratory assays. Most secure of these clinical diagnoses is that of the tropical eosinophilia syndrome; in addition to its distinctive clinical presentation such patients have extraordinarily high levels of total serum IgE (almost always in excess of 10 000 ng/ml), and their levels of specific antifilarial antibodies (both IgG and IgE) are extremely high, the absolute levels depending on the specific tests used. For other amicrofilaraemic syndromes serologic findings based on detecting IgG4 antibodies have proven helpful, since this subclass has greater diagnostic specificity and is stimulated by the presence of active infection. Such antibody analyses are also especially helpful in diagnosing the expatriate syndrome' where 'background (i.e., pre-exposure) levels' of IgG and especially IgG4 antibodies to filarial antigens will be very low, so that elevated levels have significant diagnostic implications in association with the clinical presentations.

Eosinophilia is a frequent concomitant of all filarial syndromes, but only when the levels are extremely high (as in tropical eosinophilia or the expatriate syndrome ) are they diagnostically helpful.