HIGH DOSE HOOK EFFECT IN SERUM TOTAL AND FREE PROSTATE SPECIFIC ANTIGEN IN A PATIENT WITH METASTATIC PROSTATE CANCER

High dose hook effect is a phenomenon in which samples with extremely high analyte concentrations may paradoxically produce a lower response than expected in an immunoradiometric assay. This effect has been observed while measuring serum prostate specific antigen (PSA) with a Tandem R assay (Hybritech Beckman-Coulter Corp., San Diego, California). 1 We report the occurrence of a hook effect while measuring total and free PSA in a patient with metastatic prostate cancer. CASE REPORT An 85-year-old man was referred to us for paraplegia and suspected bone metastasis of prostate cancer. Serum total and free PSA measured with a Tandem R assay before digital rectal examination was 11 and 0.5 ng./ml., respectively, and g-seminoprotein measured with an Eiken kit (Eiken, Tokyo, Japan) was 1,800 ng./ml. Digital rectal examination revealed enlargement of the left lobe with no extension beyond the prostate. Bone scintigraphy suggested metastases to the pelvis with the extent of disease score 2. Pathological examination confirmed the diagnosis of adenocarcinoma of the prostate (Gleason 4 to 5). Orchiectomy was performed simultaneously. Because we thought total PSA of 11 ng./ml. was falsely low, repeat analysis after dilution was performed. After 1,0003 dilution total and free PSA concentrations were 26,000 and 1,050 ng./ml., respectively. We determined that the hook effect had occurred in this case. Although total and free PSA values decreased, the patient died of prostate cancer 2 months after orchiectomy. DISCUSSION