Hemoccult stool tests: false-negative results due to storage of specimens.

Stoll samples mixed with 0 to 38 mg of hemoglobin per gram of stool were tested for occult blood by the Hemoccult (HO) method when fresh and after storage for 1 to 14 days. The intensity of the HO reaction in fresh stool samples was directly related to the hemoglobin concentration, and approximately 8 mg/g was required for a positive reaction. After samples had been stored on HO cards for 4 days, the intensity of the HO reaction was decidedly less in all. Except at the highest concentrations studied, all became HO-negative or trace within 8 days. In contrast, samples of most concentrations that were stored in tightly capped containers prior to preparation of HO cards maintained a positive reaction intensity for 1 week. Nevertheless, the intensity of the HO reaction decreased significantly (P less than 0.01) with each method of storage. No false-positives were induced by storage during this portion of our study. Samples from 100 stool specimens sent to the clinical laboratory for determination of occult blood were also smeared on separate HO cards for testing immediately and after 4, 7, or 10 days of storage. Only one positive HO reaction was observed on an HO card prepared from a specimen initially classified as HO-negative. In contrast, up to 40% of the stored HO cards prepared from specimens initially classified as HO-positive were HO-negative when analyzed 4 days later.