gen 1, which may indicate acute infection (Früh et al., 1989); and antibodies to circumsporozoite proteins as a broad screen for malaria prevalence (Wirtz et al., 1987). Comparison of the sensitivities and specificities of these assays has been difficult since the various protocols and antigen extracts have not been standardized.
To overcome the difficulties associated with producing a standardized, contaminant-free supply of antigen, recombinant DNA techniques have been used to identify, clone, and express or synthesize epitopes of parasite proteins. Among these are the asparagine-alanine-asparagine-proline (NANP) repeat sequences of the P. falciparum circumsporozoite protein (Del Giudice et al., 1987; Knobloch et al., 1987). Synthetic peptide antigen generated by these amino acid repeat sequences was used to develop an ELISA method that identified 80 percent of the sera shown to contain P. falciparum antibodies by RIA (Zavala et al., 1986). A commercial ELISA kit that uses NANP as the antigen has also been developed (Esposito et al., 1990).
Assays that detect antibodies are generally easier to develop than those that detect antigens. There are a number of potentially simple, inexpensive antigen-based assay systems that may be used to diagnose malaria. These include latex bead agglutination, solid-phase dipstick, membrane dot-blot, and hemagglutination. At present, these tests offer generally lower sensitivity and cost in exchange for increased speed and convenience. Although in certain instances sensitivity may be less critical than speed and ease of use, a decision to use one of these tests as a replacement for microscopy should be given careful consideration.
One of the more promising rapid diagnostic methods that should be evaluated for use in malaria is the autologous red blood cell agglutination assay (Kemp et al., 1988). The technology, developed for detecting antibodies to the AIDS virus, utilizes a functionally univalent antibody reagent that binds to but does not agglutinate human red blood cells. A selected monoclonal antibody or polyclonal antiserum is then chemically conjugated to a monoclonal antibody reagent. When mixed with whole blood, the monoclonal antibody conjugates bind to the red blood cells, which then agglutinate if antigen is present. This assay, potentially rapid, simple, and inexpensive, may also be used to detect serum antibodies in whole blood by conjugating the monoclonal antibody to a specific antigen.
The needs of those who use diagnostic tests must be considered when new assays are developed. Table 5-1 summarizes a set of test specifications for various categories of users. Biotechnology companies, university