An improved approach to estimate the avidity index of immunoglobulins: Evaluation of the method using IgG anti-Toxoplasma gondii

GARCÍA, VALERIA S.; PERETTI, LEANDRO E.; PEVERENGO, LUZ; DALLA FONTANA, MARIA L.; GUGLIOTTA, LUIS M.; GONZÁLEZ, VERÓNICA D.G.; LAGIER, CLAUDIA M.; MARCIPAR, IVÁN S.

JOURNAL OF IMMUNOLOGICAL METHODS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2018 p. 78 - 81

0022-1759

The daily clinical practice frequently force physicians to make a decision of whether a patient is coursing an infectious disease at the acute or the chronic stage, since treatment may be different depending on it. This is the case of toxoplasmosis, a worldwide disease caused by the intracellular parasite Toxoplasma gondii. When a pregnant woman is infected for the first time by T. gondii and is not treated, the parasite can cross the placenta and severely affect the fetus. In case of primary infection, the treatment protocol includes using antiparasitic drugs to prevent serious damages, including death of the unborn baby. However, antiparasitic chemicals have side effects that should be avoided, unless treatment is mandatory. That is why discrimination between long-term and primary T. gondii infection turns out to be critical in infectedpregnant women.The identification of the acute-infection state by only finding specific IgM in serum, a common procedure used for other infection diagnosis, is not reliable because residual IgM may occur for even more than one year after the first exposure to the parasite, turning its diagnosis quite difficult. IgG-avidity assays can help to differentiate between recently acquired and distant infection, by using the avidity index (AI). When diagnosing acute toxoplasmosis, although IgG maturation rate varies with each individual, a high AI confirms the chronic phase, thus ruling out a recent infection.On the other hand, a low AI suggests that the sample belongs to a recentlyinfected individual, though exceptions to this outline exist. As firstly proposed by Hedman et al., anti-T. gondii IgG-AI is estimated using two different titration curves obtained by treating each sample dilution with two different solutions, one with and the other without 6M urea. The corresponding end-points (E-P) are determined for a particular cutoff-value, and results are expressed as the percentage calculated on the basis of the E-P ratio.Nevertheless, difficulties may arise in using avidity curves when the number of dilutions is insufficient, i.e. when the highest routine dilution renders an optical density (OD) higher than the cutoff-value. An easier alternative to the E-P titration method is the OD one, where triplicate measurements of the IgG amount are performed at a single dilution level, and AI is calculated as the ratio between the mean OD obtained having washed with and without urea. The maindifficulty of this method is that AI can noticeably vary with the total amount of anti-T. gondii IgG in the specimen. Prince and Wilson combined the accuracy of assays based on the E-P titers and the simplicity of OD assays, where IgG reactivity was measured quantitatively using a standard curve.Here, we have used rP22a, an acute-phase recombinant protein we recently described, to assess anti-T. gondii IgG AI, calculated by two new easier approaches, one based on the area under the avidity curve (AUAC) and the other one based on the E-P titration method with minor variations. We compare our results with those obtained when calculating the AI by the conventional methods above mentioned, and the performance to render a proper sample classificationas compared to that obtained having used a commercial kit as standard.