An acute-phase blood sample should always be taken as soon as possible after the onset of suspected dengue illness, and a convalescent-phase sample should ideally be taken 2 to 3 weeks later. Because it is frequently difficult to obtain convalescent-phase samples, however, a second blood sample should always be taken from hospitalized patients on the day of discharge from hospital.
Serologic Diagnosis
Five basic serologic tests have been routinely used for diagnosis of dengue infection; hemagglutination-inhibition (HI), complement fixation (CF), neutralization test (NT), immunoglobulin M (IgM) capture enzyme-linked immunosorbent assay (MAC-ELISA), and indirect immunoglobulin G ELISA (
47,
55,
148). Regardless of the test used, unequivocal serologic diagnosis depends upon a significant (fourfold or greater) rise in the titer of specific antibodies between acute- and convalescent-phase serum samples. The antigen battery for most of these serologic tests should include all four dengue virus serotypes, another flavivirus (such as yellow fever virus, Japanese encephalitis virus, or St. Louis encephalitis virus), a nonflavivirus (such as Chikungunya virus or eastern equine encephalitis virus), and ideally, an uninfected tissue control antigen (
47).
Of the above tests, HI has been the most frequently used; it is sensitive, is easy to perform, requires only minimal equipment, and is very reliable if properly done (
28). Because HI antibodies persist for long periods (up to 48 years and probably longer) (
58), the test is ideal for seroepidemiologic studies. HI antibody usually begins to appear at detectable levels (titer of 10) by day 5 or 6 of illness, and antibody titers in convalescent-phase serum specimens are generally at or below 640 in primary infections, although there are exceptions (
4,
47). By contrast, there is an immediate anamnestic response in secondary and tertiary dengue infections, and reciprocal antibody titers increase rapidly during the first few days of illness, often reaching 5,120 to 10,240 or more. Thus, a titer of ≥1,280 in an acute-phase or early convalescent-phase serum sample is considered presumptive evidence of a current dengue infection. Such high levels of HI antibody persist for 2 to 3 months in some patients, but antibody titers generally begin to wane by 30 to 40 days and fall below 1,280 in most patients (
47). The major disadvantage of the HI test is its lack of specificity, which generally makes it unreliable for identifying the infecting virus serotype. However, some patients with primary infections show a relatively monotypic HI response that generally correlates with the virus isolated (
47).
The CF test is not widely used for routine dengue diagnostic serologic testing. It is more difficult to perform, requires highly trained personnel, and therefore is not used in most dengue laboratories. It is based on the principle that complement is consumed during antigen-antibody reactions (
20). CF antibodies generally appear later than HI antibodies, are more specific in primary infections, and usually persist for short periods, although low levels of antibodies persist in some persons (
47). It is a valuable test to have in a diagnostic laboratory because of the late appearance of CF antibodies; some patients thus show a diagnostic rise in antibody titers by CF but have only stable antibody titers by HI or ELISA (
47). The greater specificity of the CF test in primary infections is demonstrated by the monotypic CF responses when HI responses are broadly heterotypic; it is not specific in secondary infections. The CF test is useful for patients with current infections but is of limited value for seroepidemiologic studies, where detection of persistent antibodies is important.
The NT is the most specific and sensitive serologic test for dengue viruses (
33,
129). The most common protocol used in dengue laboratories is the serum dilution plaque reduction NT. In general, neutralizing-antibody titers rise at about the same time or slightly more slowly than HI and ELISA antibody titers but more quickly than CF antibody titers and persist for at least 48 years (
58). Because the NT is more sensitive, neutralizing antibodies are present in the absence of detectable HI antibodies in some persons with past dengue infection.
Because relatively monotypic neutralizing-antibody responses are observed in properly timed convalescent-phase serum, the NT can be used to identify the infecting virus in primary dengue infections (
4,
47,
129,
148). As noted above, the HI and CF tests may also give monotypic responses to dengue infection that generally agree with NT results. In cases when the responses are monotypic, the interpretation of all these tests is generally reliable. In secondary and tertiary infections, determining the infecting virus serotype by NT or any other serologic test is not reliable (
90). Because of the long persistence of neutralizing antibodies, the test may also be used for seroepidemiologic studies. The major disadvantages are the expense, time required to perform the test, and technical difficulty. It is therefore not used routinely by most laboratories.
MAC-ELISA has become the most widely used serologic test for dengue diagnosis in the past few years. It is a simple, rapid test that requires very little sophisticated equipment (
17,
47,
78,
89,
97). Anti-dengue IgM antibody develops a little faster than IgG antibody. By day 5 of illness, most patients (80%) in Puerto Rico whose cases were subsequently confirmed by HI on paired serum samples or by virus isolation had detectable IgM antibody in the acute-phase serum in this assay (
47). Nearly all patients (93%) developed detectable IgM antibody 6 to 10 days after onset, and 99% of patients tested between 10 and 20 days had detectable IgM antibody. The rapidity with which IgM develops varies considerably among patients. Although the dates of onset are not always recorded accurately, some patients have detectable IgM on days 2 to 4 after the onset of illness whereas others may not develop IgM for 7 to 8 days after onset (
47). This variation is also reflected in the amount of IgM produced and the length of time detectable IgM persists after infection. IgM antibody is produced by patients with both primary and secondary dengue infections and probably by persons with tertiary infections, although the response in some secondary and probably most tertiary infections is low level and transient (
89). IgM antibody titers in primary infections are significantly higher than in secondary infections, although it is not uncommon to obtain IgM titers of 320 in the latter cases (
47). In some primary infections, detectable IgM persists for more than 90 days, but in most patients, it has waned to an undetectable level by 60 days. A small percentage of patients with secondary infections have no detectable IgM antibody (
89).
MAC-ELISA with a single acute-phase serum sample is slightly less sensitive than the HI test with paired serum samples for diagnosing dengue infection (
47). However, it has the advantage of frequently requiring only a single, properly timed blood sample. In one series of 288 patients during the 1986 epidemic in Puerto Rico, paired blood samples were tested by HI and the single acute-phase sample from the same pairs were tested by MAC-ELISA. The HI test on the pairs indicated that 228 (79%) were considered positive, while MAC-ELISA on the single samples indicated that 203 (70%) were positive. Five samples (1.7%) showed a false-positive response and 30 samples (10%) showed a false-negative response by MAC-ELISA (
47). When one considers the difficulty in obtaining second blood samples and the long delay in obtaining conclusive results from the HI test, this low error rate would be acceptable in most surveillance systems. It must be emphasized, however, that because of the persistence of IgM antibody for 1 to 3 months, MAC-ELISA-positive results obtained with single serum samples are only provisional and do not necessarily mean that the dengue infection was current (
47,
148). These results do mean that it is reasonably certain that the person had a dengue infection sometime in the previous 2 to 3 months. Similarly, a negative result with an acute-phase sample may be a false-negative result because the sample was taken before detectable IgM appeared. Unfortunately, many dengue diagnostic laboratories have adopted MAC-ELISA as a confirmatory test and do not conduct follow-up tests to confirm the presumptive IgM results. As noted above, this may be acceptable for surveillance reports, but it is unacceptable in a clinical setting. If this test is used to make patient management decisions, it could result in a higher case fatality rate among patients with false-negative results.
The specificity of MAC-ELISA is similar to that of HI. In both primary and secondary dengue infections, some monotypic responses may be observed, but in general, the response is broadly reactive among both dengue virus and other flavivirus antigens. With serum samples from patients with other flavivirus infections such as Japanese encephalitis, St. Louis encephalitis, and yellow fever, however, the response is generally more specific; while there may be some cross-reaction with dengue antigens, most specimens show relatively monotypic IgM responses to the infecting flavivirus (
47). In dengue infections, monotypic IgM responses frequently do not correlate with the virus serotype isolated from a patient. Therefore, MAC-ELISA cannot be reliably used to identify the infecting virus serotype.
MAC-ELISA has become an invaluable tool for surveillance of dengue, DHF, and DSS. In areas where dengue is not endemic, it can be used in clinical surveillance for viral illness or for random, population-based serosurveys, with the certainty that any positive results detected indicate recent infections (within the last 2 to 3 months). A properly timed serosurvey by MAC-ELISA during an epidemic can determine very quickly how widespread transmission has become. In areas where dengue is endemic, MAC-ELISA can be used as an inexpensive way to screen large numbers of serum specimens with relatively little effort. It is especially useful for hospitalized patients, who are generally admitted late in the illness after detectable IgM is present in the blood (
47), but it must be emphasized again that this test should not be used to make patient management decisions.
An indirect IgG-ELISA has been developed that is comparable to the HI test and can also be used to differentiate primary and secondary dengue infections (
27). The test is simple and easy to perform and is thus useful for high-volume testing. The IgG-ELISA is very nonspecific and exhibits the same broad cross-reactivity among flaviviruses as the HI test does; therefore, it cannot be used to identify the infecting dengue virus serotype. However, it has a slightly higher sensitivity than the HI test. As more data are accumulated on the IgG-ELISA, it is expected to replace the HI test as the most commonly used IgG test in dengue laboratories.
A number of commercial test kits for anti-dengue IgM and IgG antibodies have become available in the past few years. Unfortunately, the accuracy of most of these tests is unknown because proper validation studies have not been done. Some evaluations have been published (
91,
96,
146,
153), but the sample sizes have been too small to accurately measure sensitivity and specificity. Moreover, the samples generally used have represented only strong positives and negatives, with few samples representing optical densities or positive-negative values in the equivocal range. One exception to this were kits that were independently evaluated at CDC; both IgM and IgG test kits had a high rate of false-positive results compared to standard tests, especially with samples with optical densities in the equivocal range (
91). Other studies, however, have given results comparable to those of standard tests (
96,
146,
153). It is anticipated that these test kits can be reformulated to make them more accurate, making global laboratory-based surveillance for dengue and DHF an attainable goal in the near future.