Window period of anti-hepatitis A virus immunoglobulin M antibodies in diagnosing acute hepatitis A

Background The diagnosis of acute hepatitis A virus (HAV) infection is made on the basis of the presence of anti-HAV immunoglobulin M (IgM) antibodies in patients with clinical features of acute hepatitis. Some patients show a negative serology at initial presentation, which may complicate the diagnosis of hepatitis A (HA). The aim of this study was to examine the characteristics of HA patients with an initially negative anti-HAV IgM test result. Materials and methods Patients with symptomatic acute hepatitis who underwent IgM anti-HAV testing at a single center were enrolled consecutively, with tests repeated in patients with negative initial serology. Results A total of 684 patients with acute hepatitis were tested, of whom 620 patients were initially or eventually diagnosed with HA. Anti-HAV IgM was initially negative in 67 of the 620 HA patients (10.9%), but was later confirmed by subsequent retests. These patients had on average a shorter time lapse from the onset of symptoms to the initial test, a higher rate of fever, and lower alanine aminotransferase and bilirubin levels compared with those with a positive initial serology. Cutoff index (COI) values of anti-HAV IgM were correlated positively with the duration of time from the onset of symptoms to the initial test. Fever, lower bilirubin levels, and higher COI values were predictive of seroconversion to anti-HAV positivity in patients with a negative initial serology. Conclusion Taking into account the window period of HAV infection, anti-HAV IgM tests should be repeated, particularly in patients with features of the initial phase of hepatitis and a high COI value of anti-HAV IgM.