Mycoplasma pneumoniae Infections: Diagnosis Based on Immunofluorescence Titer of IgG and IgM Antibodies

Laboratory Medicine Mycoplasma pneumoniae Infections: Diagnosis Based on Immunofluorescence Titer of IgG and IgM Antibodies Mycoplasmas are small bacteria (0.3 to 0.8 //m in diameter and up to 150 μπι in length) that are unique because they have no cell wall. Two genera, Mycoplasma and Ureaplasma (associated with genital infections), are of clinical importance to humans. M. pneumoniae is primarily a respira­ tory tract pathogen that involves the nasophar­ ynx, throat, trachea, bronchi, bronchioles, and alveoli. Symptomatic infections attributable to this organism most commonly occur in children and young adults (ages 2 to 19 years). The organ­ ism has also been associated with infections of the cardiovascular, dermatologic, and central ner­ vous systems. Other mycoplasmas may be pres­ ent, especially in the upper respiratory tract; however, they are considered part of the normal microbial flora of the oral cavity and do not cause symptoms or disease. Epidemiologie Factors.—Infections with M. pneumoniae occur worldwide throughout the year, especially in temperate climates. Epidemics occur at 4- to 6-year intervals in both civilian and military populations. 1,2 The infection often seems to be a sporadic, endemic illness in families or closed communities because it has a relatively long incubation period (2 to 15 days), because its prolonged shedding in nasal secretions may result in additional infections during a protracted inter­ val, and because asymptomatic infections are common. Diagnosis.—M. pneumoniae infections may be diagnosed by cultivation of the organism on complex agar medium or by serologic methods. Because of the slow growth of the organism, partly reflecting fastid­ ious nutrient requirements, culture techniques have been expensive, tedious, and somewhat unproduc­ tive. In the clinical microbiology laboratory at the Mayo Clinic, the rate of isolation of the organism has ranged from 0 to 43% from throat and sputum

Address reprint requests to Dr. T. F. Smith, Department of Laboratory Medicine, Mayo Clinic, Rochester, MN 55905. Mayo Clin Proc 61:830-831, 1986

specimens in individual years. 3 The time interval between isolates, however, may be several months because of the sporadic occurrence of these infec­ tions. Moreover, the last five isolates of M. pneumo­ niae in our laboratory necessitated more than 20 days of incubation time before a report could be made. Perhaps genetic probes could produce more timely results with acceptable sensitivity and speci­ ficity relative to cultivation of the organism. Indirect Immunofluorescence Test.—Titra­ tion of antibodies in acute and convalescent phase serum specimens is usually done with use of the complement fixation reaction. Currently, however, cells that contain M. pneumoniae antigenic sub­ strate are commercially available (Zeus Technolo­ gies, Inc., Raritan, New Jersey), and these facilitate performance of the indirect immunofluorescence test4 and separate measurement of antibodies of IgG and IgM classes. Of 14 paired (acute and convales­ cent phase) serum specimens, in each pair of which the presence of an M. pneumoniae infection was confirmed by a rising titer of complement fixation, 11 had fourfold or greater increases in titer, as mea­ sured by the indirect immunofluorescence test. One of the three paired serum specimens that did not have a diagnostic serologic increase in titer had IgM class antibody present in both acute and convales­ cent phase serum samples. Importantly, IgM anti­ bodies, indicative of a current infection, were demon­ strated in 13 of the 14 patients. In five patients, IgM antibodies were detected in the acute phase serum sample. Twenty serum specimens submitted for cytomegalovirus serology were tested for antibodies to M. pneumoniae by the indirect immunofluorescence test. Seventeen of the 20 specimens (85%) had low levels of IgG antibody directed against the organ­ ism; of these, 15 had titers of 1:10 or less. None was positive for IgM antibodies to M. pneumoniae. Expo­ sure to this organism is widespread—in one analy­ sis, more than 90% of asymptomatic, apparently well, Mayo Clinic blood donors had IgG antibodies to M. pneumoniae, as detected by the indirect immuno­ fluorescence test (Fig. 1).

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body) infections. Nonetheless, examination of both acute and convalescent phase (7- to 10-day) serum specimens is urged, because the IgM antibody may not be demonstrable early in some cases and a rising titer confirms the diagnosis of M. pneumoniae infection. Thomas F. Smith, Ph.D. Department of Laboratory Medicine

Fig. 1. Prevalence of IgG antibody to Mycoplasma pneumoniae in 100 Mayo Clinic blood donors, as measured by indirect immunofluorescence (ImF) test. In every case, the IgM antibody was undetectable. Thus, in no case was the infection acute or recent.

The indirect immunofluorescence test for M. pneu­ moniae is sensitive and specific and can distinguish between current (IgM antibody) and past (IgG anti-

REFERENCES 1. Denny FW, Clyde WA Jr, Glezen WP: Mycoplasma pneumo­ niae disease: clinical spectrum, pathophysiology, epidemiology, and control. J Infect Die 123:74-92,1971 2. Noah ND, Urquhart AM: Epidemiology of Mycoplasma pneumoniae infection in the British Isles, 1974-9. J Infect 2:191-194,1980 3. Dorman SA, Wilson DJ, Smith TF: Comparison of growth of Mycoplasma pneumoniae on modified New York City and Hayflick media. Am J Clin Pathol 79:235-237,1983 4. Carter JB, Carter SL: Acute-phase, indirect fluorescent antibody procedure for diagnosis of Mycoplasma pneumoniae infection. Ann Clin Lab Sei 13:150-155,1983