There is a wide variety of assays available for the detection of toxoplasma-specific IgM¹.  The different sensitivities of these assays can make interpretation of the results difficult and cause problems for patient management.  Some ELISA assays detect IgM > 1 year after infection; others for 6 months.  ISAGA assays remain positive for > 1 year.  In addition, there is considerable variation in the length of time specific IgM is produced by different patients.  Different IgM tests are appropriate in different clinical situations and it is important that results are interpreted with caution. 

Too sensitive an assay may wrongly suggest infection during pregnancy.  This problem was demonstrated in a retrospective cohort study of 811 consecutive pregnant women in the USA where the rate of unnecessary abortions could be reduced by approximately 50% in women who were toxoplasma specific IgM positive if confirmatory serological IgM testing was carried out at a Reference Laboratory².  Current infections should always be confirmed by a second IgM test and interpretation should take account of dye test and specific IgG results.  A repeat sample after 10-14 days can help give a more accurate assessment of the stage of infection.

In some clinical situations a very sensitive assay is necessary for the detection of toxoplasma specific IgM e.g. in the diagnosis of congenital infection and reactivated infection in the immunocompromised.  Even with a sensitive test like the ISAGA, tests may not be diagnostic.  Detection of specific neonatal IgM will be negative in some infected babies.  IgM can be detected in some reactivated infections.

All toxoplasma specific IgM tests produce some false positive and false negative results.  Samples should be referred to a Reference Laboratory when the IgM test is positive for confirmation; or when the IgM test is negative but  there is a strong clinical suspicion of toxoplasmosis.

References

1. Evans R, Ashburn D, Chatterton J, Joss A, Ho-Yen DO (2002).  How to detect current  toxoplasma infection.  British Journal of Biomedical Science 59(1), 4-6.

2. Liesenfield O, Montoya JG, Tathineni NJ et al (2001).  Confirmatory serologic testing for acute toxoplasmosis and rate of induced abortions among women reported to have positive toxoplasma immunoglobulins M antibody titres.  Am J Obstet Gynaecol 184, 140-145.