Tympanometry provides useful quantitative information about the presence of fluid in the middle ear, mobility of the middle ear system, and ear canal volume. Its use has been recommended in conjunction with more qualitative information (e.g., history, appearance, and mobility of the tympanic membrane) in the evaluation of otitis media with effusion and to a lesser extent in acute otitis media. It also can provide useful information about the patency of tympanostomy tubes. Tympanometry is not reliable in infants younger than seven months because of the highly compliant ear canals of infants. Tympanogram tracings are classified as type A (normal), type B (flat, clearly abnormal), and type C (indicating a significantly negative pressure in the middle ear, possibly indicative of pathology). According to the Agency for Healthcare Research and Quality guidelines on otitis media with effusion, the positive predictive value of an abnormal (flat, type B) tympanogram is between 49 and 99 percent. A type C curve may be useful when correlated with other findings, but by itself it is an imprecise estimate of middle ear pressure and does not have high sensitivity or specificity for middle ear disorders.