In the Toxoplasma IgG distribution in October 2005 two samples 7672 and 7673 caused problems for some participants. Both specimens were undiluted and came from single donations
and were positive by dye test in pre and post distribution testing (dye test 8 IU/mL and 16 IU/mL respectively). Specimen 7672 was reported positive by 56% of participants with 33% reporting an equivocal result
and 11% a negative result; while 67% of participants reported specimen 7673 as positive, 24% as equivocal and 9% as negative. All kits were affected.These samples highlight the different sensitivities,
specificities and thresholds of the many different tests used for toxoplasma testing and encourage participants (and manufacturers) to evaluate the performance of their kits and their testing strategy. The likely cause
of problems was the low level of specific antibody present. However, detection of low levels of antibody can be important for patient management.
Difficulties in immunity testing have been highlighted in a
previous teaching sheet (January 2004). Baseline immunity testing is recommended for immunocompromised patients (especially HIV and bone marrow transplant) to identify past toxoplasma infection. This ensures that
patients at risk of reactivated toxoplasma infection, which can be life threatening, are identified and given appropriate prophylaxis. Failure to detect low level specific toxoplasma antibody would give false
reassurance that there is no risk of reactivated toxoplasma infection with serious consequences for patient management. We recommend a threshold equivalent to 2 IU/mL in dye test in these circumstances.
Samples with low level toxoplasma antibody are a reality and this should be reflected in the samples in the NEQAS distributions. It is the intention that such samples will be included occasionally and
they will be scored providing the results are confirmed by pre and post distribution testing. It is recognised that samples of this kind will represent a more stringent challenge to accurate diagnosis but they will also
be a valuable teaching tool. A better understanding of how each laboratory's tests perform will improve patient management.
