Immunoenzymatic method in homogeneous phase for the determination of anti-plasmodium falciparum-sporozoite antibodies in human blood

Immuno-enzymatic method for the determination of anti-P. falciparum-sporozoite antibodies in a sample of blood, which comprises:

• (a) separately incubating two aliquots, (i) and (ii), of a same sample with a synthetic antigen-enzyme conjugate (P-E), wherein the synthetic antigen (P) of the conjugate is capable of reacting with anti-P. falciparum-sporozoite antibodies present in the sample to form an antisporozoite-synthetic antigen-enzyme complex and the enzyme (E) of the conjugate is capable of catalyzing a colormetric reaction, in phosphate buffer at a pH value within the range of from 6.5 to 8.00, in the presence of a non-ionic and/or weak anoinic detergent at a concentration now lower than 0.05% (weight/volume or volume/volume) (w/w or v/v), and at a temperature within the range of from 15.degree. C. to 35.degree. C., wherein one aliquot (i) of the sample has been previously admixed with the same synthetic antigen (P) not labeled with the enzyme at such a concentration as to completely inhibit the formation of the antisporozoite antibody-synthetic antigen-enzyme complex (Ab-P-E);
• (b) adding to the samples treated as reported in step (a), PEG 6000 as an inert substance capable of quantitatively precipitating the antisporozoite antibody-synthetic antigen-enzyme complex at an end concentration within the range of 13% to 17% (W/V);
• (c) separating precipitate from the reaction mixtures obtained in step (b), either by filtration by centrifugation;
• (d) repeatedly washing precipitate obtained in step (c) with a buffer solution to which the PEC 6000 is added, up to an end concentration within the range of from 13% to 17% (W/V);
• (e) dissolving the so obtained precipitate in a buffer solution at a small pH value within the range of from 4.0 to 5.5, containing a non-ionic and/or weak anionic detergent at an end concentration not lower than 0.05% (w/v or v-v), in the presence or not of the PEG 6000, at an end concentration within the range of from 3% to 5% (w/v);
• (f) adding to the so obtained solutions, a colorless enzymatic substrate, specific for the enzyme used and capable of generating, at a pH within the range of 4.0 to 5.5, in case the sample under examination is positive, a colored product;
• and
• (g) finally determining the intensity of the color by comparative absorbance spectrophotometry between aliquot (i) and aliquot (ii).