A 52-year-old female reports black stools. What is the most appropriate initial diagnostic study?

The most appropriate initial diagnostic study for a patient presenting with black stools is a stool for occult blood. This test helps to determine whether there is hidden (occult) blood in the stool, which can indicate gastrointestinal bleeding. Black stools, also known as melena, often suggest an upper gastrointestinal source of bleeding, such as from a peptic ulcer or varices. By first performing the occult blood test, healthcare providers can assess the presence of blood in the stool, which can guide further diagnostic evaluation and management.

In contrast, stool cultures are primarily used to identify infectious organisms in cases of diarrhea or gastrointestinal infections, and they are not indicated in the initial assessment of black stools. Sigmoidoscopy involves direct visualization of the rectum and sigmoid colon, but it is not typically the first step in diagnosing the cause of melena since the source of bleeding may be higher in the gastrointestinal tract. An abdominal CT scan may be useful in certain situations but is not usually the first-line diagnostic tool for investigating black stools. Starting with a stool for occult blood test provides a non-invasive and effective means of evaluating for potential gastrointestinal bleeding.