Interpreting serological findings after transfusion or administration of blood products

The serological response to toxoplasma infection in the immunocompetent follows the classic pattern of appearance of acute-phase immunoglobulins (IgM and IgA) followed by relatively high levels of IgG.  After several months IgM and IgA levels will fall until these can no longer detected, while IgG will fall to a resting level that will persist for decades or for life. 

However, where the patient is immunosuppressed or immunodeficient, the typical primary immune response may be partially or fully abrogated and seroconversion may be limited to production of lower levels of IgG.  Thus, 'seroconversion' from negative to low levels of IgG in a patient with the appropriate clinical circumstances will often lead to toxoplasmosis being placed high among the list of differential diagnoses.  Similarly, an increase in IgG levels may be interpreted as evidence in support of reactivation of infection and, again, treatment may be considered.

Such 'seroconversion' or rising IgG levels may indeed be the result of a recently-acquired or reactivated toxoplasma infection.  However, where the patient may have received blood transfusions or blood products containing immunoglobulins, consideration should be given to the possibility that these may be the source of the anti-toxoplasma IgG detected.  The importance of considering such products as a potential source is underlined by the fact that the prevalence of toxoplasma infection among the UK blood donor population ranges from approximately 10-30%.

Thus, while both the appearance of IgG or subtle increases in level may be clinically significant in the immunosuppressed and immunodeficient, these should be interpreted with caution where blood products have been administered.  Additional tests that may be helpful in discriminating novel and transfusion-associated IgG include measurement of IgG avidity (anti-toxoplasma IgG in the blood donor population will be almost exclusively from infections acquired greater than 6 months prior to donation) or investigation for active toxoplasmosis using PCR.