Situation: A 58-year-old man who has fished for a living since age 15 comes to the clinic because of a dark, irregular patch on the sole of his left foot. Melanoma with lymph node spread is confirmed, and he later receives pembrolizumab, an immune checkpoint inhibitor. Two weeks after his third dose, he reports six watery stools a day with cramping. What should the nurse do?

Checkpoint inhibitors release immune brakes and can cause immune-related colitis weeks to months after a dose. New diarrhea must be reported early because treatment usually requires corticosteroids and holding the drug. Treating it at home as ordinary diarrhea delays care of a potentially severe inflammation.

Core clinical reasoning
Pembrolizumab is a programmed cell death 1 (PD-1) inhibitor. By blocking the PD-1 pathway, it removes the normal brake on T-cell activation, which enhances antitumor immunity but can also trigger autoimmunelike inflammation in healthy tissues ^[2]. When the gastrointestinal tract is affected, the result is immune-mediated diarrhea and colitis (IMDC), which typically presents as watery, frequent stools with cramping and can appear weeks to months after a dose ^[2][4]. This is not ordinary chemotherapy-induced diarrhea; it is an immune-related adverse event that can progress to severe colitis, perforation, or systemic illness if not recognized early ^[3].

New or worsening diarrhea in a patient receiving an immune checkpoint inhibitor must be reported to the oncology team promptly, not managed at home as routine diarrhea. Early intervention usually requires holding the drug and starting corticosteroids, and delays in recognition are associated with worse outcomes ^[3][4]. The patient’s six watery stools per day with cramping after his third dose is a clinically significant change that warrants immediate communication.

Assessment focus	Why it matters	Nursing action
Timing	IMDC can occur weeks to months after any ICI dose, so a new symptom after the third dose is highly suspicious	Do not assume it is unrelated to pembrolizumab
Stool pattern	Watery stools with cramping suggest colonic inflammation rather than simple increased motility	Assess frequency, blood, mucus, fever, abdominal pain, and hydration status
Severity	Six stools per day is beyond mild; grading guides whether corticosteroids and drug interruption are needed	Report exact frequency and associated symptoms to the oncology team

Watch out! Loperamide may mask symptoms and delay definitive treatment of immune-mediated colitis. It is not the first-line nursing response when checkpoint-inhibitor colitis is suspected.
Key point! Immune-related diarrhea is managed with early recognition, drug holding, and corticosteroids, not with routine antidiarrheal advice or watchful waiting.

The rationale for early reporting is supported by evidence that early endoscopic evaluation and prompt immunosuppressive therapy improve outcomes in patients with IMDC ^[4]. Fecal microbiota transplantation is reserved for refractory cases and is not an initial nursing action . Therefore, the nurse should report the diarrhea to the oncology team promptly so that grading, stool infection workup, and corticosteroid initiation can occur without delay ^[2][3].