The difference between positive and negative tubes used in our rapid test for syphilis1 may seem sometimes of insufficient sharpness to the inexperienced eye. The source of difficulty is the strong Tyndall phenomenon exhibited by the nonflocculated antigen sometimes complicating the interpretation of results.2
This possible source of error can be easily eliminated by centrifugation of the test tubes, thus increasing the size of flocculi, before the results are recorded. Very light tubes which do not have to be balanced are preferable for this manipulation.
Though a useful procedure, centrifugation is by no means essential, and clearcut results are obtained when the tubes are inspected directly under suitable conditions of illumination. The well known principle of viewing the tubes under indirect illumination against a dark background is employed. This can be accomplished with roentgen ray film wrappers will do),
RAPPAPORT F, EICHHORN F. AN ILLUMINATING MAGNIFIER FOR THE READING OF THE RAPID TEST FOR SYPHILIS. Arch Derm Syphilol. 1946;54(3):345–347. doi:10.1001/archderm.1946.01510380092012
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