False negative Guaiac stool

Q: 42 years old male with known history of esophageal varices is admitted to ICU with bloating, 'loin to groin,' and abdominal pain. Due to the associated history, GI bleed is suspected. Patient reported recent 'cold-like symptoms' and was consuming large doses of vitamins to improve his immunity. Bedside stool guaiac turns negative. Toxicity of which vitamin may give false-negative stool guaiac?

Answer: Vitamin C

Unfortunately, vitamin C is largely abused in the population. Mega doses of vitamin C can result in false-negative stool guaiac. Other symptoms are diarrhea, abdominal pain, and bloating. Moreover, it may give rise to oxalate kidney stones, particularly in males. There are some reports of association with fatal cardiac arrhythmias when mega doses of vitamin C and iron are taken together.

#toxicology

References:

1. Jaffe RM, Kasten B, Young DS, MacLowry JD. False-negative stool occult blood tests caused by ingestion of ascorbic acid (vitamin C). Ann Intern Med 1975; 83:824.

2. Ferraro PM, Curhan GC, Gambaro G, Taylor EN. Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones. Am J Kidney Dis 2016; 67:400.

3. McLaran CJ, Bett JH, Nye JA, Halliday JW. Congestive cardiomyopathy and haemochromatosis--rapid progression possibly accelerated by excessive ingestion of ascorbic acid. Aust N Z J Med 1982; 12:187.