A 67-year-old woman prefers stool testing for blood over colonoscopy for cancer screening. What is the physician most concerned about regarding the stool test?

The primary concern regarding the stool testing for blood as a method of cancer screening pertains to its low sensitivity. Stool tests, such as fecal occult blood tests (FOBT) or fecal immunochemical tests (FIT), are designed to detect blood in the stool, which can be a sign of colorectal cancer. However, these tests may miss cancerous lesions, especially in cases where there is intermittent bleeding or when the lesions are small. This means that not all individuals with colorectal cancer will test positive on these stool tests, leading to a higher chance of missing cases of cancer when compared to the direct visualization and sampling capabilities of colonoscopy.

In contrast, while stool tests have a relatively good specificity for detecting blood, the concern is primarily about the ability to correctly identify all cases of cancer rather than mistakenly identifying positive cases.

Consequently, low sensitivity can result in false-negative results, where a person with colorectal cancer might receive a negative test result based on the stool test, potentially leading to delayed diagnosis and treatment. Thus, the physician's main concern when a patient opts for stool testing for cancer screening is indeed its low sensitivity.