The 52-year-old truck driver in the office of Marvin M. Lipman, Consumer Reports’ chief medical adviser, had rushed in a few minutes earlier. He had passed a large black stool (well known as an indicator of internal bleeding) and feared the worst. A test for the presence of blood was negative, so Lipman proceeded to ask him his usual questions when confronted with black stools devoid of blood.

Had he taken Pepto-Bismol, or anything else containing bismuth, to settle an upset stomach? Was he taking iron supplements? Did he eat a lot of beets, blueberries or caviar? The patient’s answer was “no” to each, but then he volunteered the following: “Could it have been the squid-ink pasta I ate last night?” Yes!

Lipman added squid-ink pasta to his list of substances that cause non-worrisome dark stools.

That case ended happily, but the result could just as well have been otherwise. Had that black stool actually contained blood, the patient’s observation might have been lifesaving, leading to early treatment of whatever the cause was. It may not be the most pleasant task, but checking your stool is important because changes in its shape, consistency or color can be a sign of an underlying condition, and your description could help your doctor diagnose the cause. If you notice any of these warning signs, give your doctor a call.

If it floats . . . Most stools sink. However, if they contain fat they may float, and you may see actual fat droplets on the water’s surface. A lack of pancreatic enzymes may be to blame. That can occur as a result of chronic inflammation of the pancreas or following its removal. Floating stools can also be a sign of intestinal malabsorption from diseases that affect the intestinal wall, such as celiac disease. The prescription weight-loss drug Xenical and its over-the-counter version, Alli, can also cause fatty stools because they block fat absorption to some degree.

(Sean McCormick for The Washington Post)

If it’s pencil-thin . . . The occasional thinner-than-normal stool is usually not a concern. Repeated pencil-thin ones, however, indicate something is obstructing or constricting the rectum. Such narrowing can result from spasm of the rectal wall due to internal hemorrhoids, inflammation from radiation treatments or colitis. A very enlarged prostate might also narrow the passageway; a cancer in the rectum is also a possibility.

If it’s the color of pale clay . . . Bile made by the liver and concentrated in the gallbladder is responsible for the medium-brown color of normal stool. A lack of color implies a blockage of the flow of bile from the liver to the intestine. The culprit may be a gallstone blocking a bile duct. Other causes include hepatitis and cancer of the ducts or the pancreas.

If it has mucus in it . . . A small amount of visible mucus in stool is normal. (The clear jellylike substance is found throughout the digestive tract.) But stool containing large globs of mucus points to a problem. That could be an indicator of an inflammatory bowel disease, such as mucus colitis, or it could indicate a mucus-secreting intestinal polyp.

If it’s mushy, pasty and similar to diarrhea . . . Mushy or watery stools that come with frequent trips to the toilet — and are often accompanied by fever — are typical signs of infection with Clostridium difficile, a type of bacteria that lives in many people’s intestines, where it is usually kept in check by other intestinal bacteria. But a prolonged course of antibiotics — especially broad-spectrum ones such as clindamycin (Cleocin and related generics) — can kill off those other intestinal bacteria, allowing C. diff to grow out of control. C. diff releases a toxin that attacks the intestinal wall; if left untreated, it can be life-threatening. Metronidazole (Flagyl and generics) or vancomycin (Vancocin and generics) are required to quell the superinfection. Even so, there is a substantial recurrence rate.

Copyright 2014. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.