Academic journal article Bulletin of the World Health Organization

The Past and Present Role of the Sabin-Feldman Dye Test in the Serodiagnosis of Toxoplasmosis

Academic journal article Bulletin of the World Health Organization

The Past and Present Role of the Sabin-Feldman Dye Test in the Serodiagnosis of Toxoplasmosis

Article excerpt

Voir page 933 le resume en francais. En la pagina 934 figura un resumen en espanol.

Introduction

The serodiagnosis of Toxoplasma gondii infection is widely used for screening pregnant women in order to prevent its congenital spread. The Sabin-Feldman dye test was the first test system able to detect specific antibodies to Toxoplasma gondii at low levels and to differentiate acute and latent infection (1). The test is based on complement-mediated cytolysis of antibody-coated live T. gondii tachyzoites, which is indicated by their inability to take up methylene blue. Selection of the dye is of prime importance in the performance of the test, and it was observed that the immunity phenomenon was inducible only when a so-called accessory factor from human serum was present. Sabin & Feldman suspected that the action of the accessory factor was complement-like (1), but the nature of the activator system was only resolved by Schreiber & Feldman (2) when they described C2 as the active factor in human serum. The finding of a heat-labile, anti-Toxoplasma factor led to the recommendation that all specimens should be heat-inactivated. This factor was later referred to by Jettmar (3) as Toxoplasma-hostile factor.

Westphal & Knuttgen (4) were the first to question the specificity of the dye test. Its specificity could not be proved as long as the comparatively low-sensitive complement fixation test (egg antigen) was used as the only reference. In 1954, the dye test became reproducible in the USA (blind comparative tests) but was relatively unstandardized in other countries (5). Test performance and interpretation were subsequently standardized (6).

A modification of the dye test involving the use of microtitration plates was developed in 1966 (7). It required a smaller quantity of parasites and activator serum but did not overcome the difficulties of using the test in routine serology. The development of the immunofluorescence, agglutination, and haemagglutination tests (8, 9), followed by the enzyme-linked immunosorbent assay (10-12), led to commercial test systems for T. gondii-specific antibodies.

Fifty years after its introduction, the dye test is still in use and is regarded as the reference test with the highest sensitivity and specificity. By detecting IgM, IgA and IgG antibodies (total immunoglobulins), it permits both very early and late diagnosis of T. gondii infection in human and animal sera. A first step towards the standardization of toxoplasmosis serology was taken in 1968 when WHO recommended the expression of dye test titres in international units per millilitre (IU/ml) (13). A first international standard serum for total anti-Toxoplasma antibodies was produced, and a second international standard followed in 1980 (14). The estimation of IU in both preparations is based on dye test results or the detection of all Ig classes using membrane antigens on the surface of live T. gondii.

At present, fully automated commercial test systems are replacing conventional in-house assays and increasingly fewer laboratories in Europe (15) and the USA (16) perform the dye test routinely.

Interlaboratory modifications in the test performance and possible antigen variations of the original T. gondii isolates within the past 50 years have raised concern as to whether the results remain comparable. The aim of the present study was to evaluate the Sabin-Feldman dye test in a multicentre study.

Materials and methods

Participants. Eighteen centres in seven European countries and one laboratory in Israel were identified as routinely performing the Sabin-Feldman test (Table 1). The laboratories received a code number and the study was performed anonymously.

Table 1. Dye test performance and interpretation

Participant    Toxoplasma         Primary       Dilution
               strain           dilution(a)      step(b)

Austria
  D 12         RH                   1:5           2; 10
  D 29         RH                   1:4             4

Germany
  D 80         BK                   1:4             4
  D 42         BK                   1:4             4
  D 30         RH                   1:16            4
  D 2          BK                   1:4             4
  D 68         BK                   1:4             4
  D 27         RH                   1:4             4

France
  D 9          RH                   1:10            2
  D 53         RH                   1:4             2
  D 15         RH                   1:2             2

United
Kingdom
  D 18         RH                   1:16            2
  D 36         RH                 1:8, 1:2          2
  D 71         RH                   1:4             2

Denmark
  D 74         RH                   1:10            4

Norway
  D 48         RH                Undiluted          2

Italy
  D 19         RH                   1:4             2
  D 66/25      C-56                 1:4             2

Israel
  D 5          RH                   1:4             2

Participant    Result       Cut-off(c)

Austria
  D 12         Titre          1:10
  D 29         Titre          1:4

Germany
  D 80         Titre          1:16; 4 IU
  D 42         Titre          1:16
  D 30         Titre          1:16
  D 2          Titre          1:4
  D 68         Titre          1:16
  D 27         Titre          1:16

France
  D 9          IU             1:8; 2 IU
  D 53         IU             >4 IU
  D 15         IU             >4 IU

United
Kingdom
  D 18         IU             4 IU
  D 36         IU             8 IU
  D 71         Titre          1:4; 2 IU

Denmark
  D 74         Titre          1:10

Norway
  D 48         IU             > 6 IU

Italy
  D 19         Titre; IU      >8 IU
  D 66/25      Titre; IU      >1:32;
               >51U

Israel
  D 5          IU             1:16; 4 IU

(a) First serum dilution tested. …

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