Large discrepancy between prostate-specific antigen results from different assays during longitudinal follow-up of a prostate cancer patient.
A case is presented of a patient with stage D prostatic carcinoma, from whom a serum sample proved to be an outlier in a correlation study performed with a 2nd-generation prostate-specific antigen (PSA) assay on the Immulite system (6.4 micrograms/L) and IMx (101 micrograms/L). Clearly, the PSA result reported by Immulite was falsely low. For nine longitudinal samples, Immulite results were approximately 20-fold lower than the IMx value (range of IMx results 5-275 micrograms/L). A selection of the samples was analyzed with the ACS:180, ES-600, and IMx (all > 180 micrograms/L); Immulite, DPC Coat-A-Count IRMA, Immuno 1, AIA-pack, and Tandem-R (all <70 micrograms/L); and Immulite free PSA assay (41 micrograms/L). Gel filtration demonstrated that apart from the alpha1-antichymotrypsin (ACT) complex, no other complexes were found. However, the sample consisted of 53% free PSA (IMx). Possibly, a change of conformation of the PSA molecule resulted in a decreased binding to ACT and a reduced affinity of the antibodies used in the affected assays.
Van Duijnhoven
H L
HL
Centralized Department of Clinical Chemistry and Hematology, St. Elisabeth Hospital, Tilburg, The Netherlands.
Péquériaux
N C
NC
Van Zon
J P
JP
Blankenstein
M A
MA
D000368:Aged; D005188:False Negative Reactions; D006801:Humans; D007118:Immunoassay / Q000706:statistics & numerical data*; D008137:Longitudinal Studies; D008297:Male; D017430:Prostate-Specific Antigen / Q000097:blood*; D011471:Prostatic Neoplasms / Q000097:blood*; D000514:alpha 1-Antichymotrypsin / Q000097:blood
D000514:alpha 1-Antichymotrypsin; D017430:Prostate-Specific Antigen