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The “Corn Test” for Constipation is Worthless

By Steve Hodges, M.D.

corn test for constipation

I met with a patient this week — an 8-year-old boy with both daytime and nighttime wetting — who had been advised by another physician to check for constipation using the “corn test.” I told the boy and his dad what I always tell folks about the ever-popular corn test: Don’t bother.

Yes, the corn test is easy, noninvasive, and costs nothing — well, besides the corn. You simply eat corn and note how long it takes for the bright yellow kernels to show up in your stool. (Corn is recommended because it doesn’t get fully digested and is easy to identify in poop.)

The number of hours is considered the corn’s “transit time.” My patient’s dad told me, “Our urology clinic said that one day is normal — anything longer is an issue.”

This is misleading!

While it’s true that everyone should poop daily, you can’t assume a child who poops every day is not constipated. Bowel transit time is just not a reliable indicator of constipation. A child who is “regular” can nonetheless have a rectum chock full of stool. In fact, many severely constipated children poop two or three times a day.

How can this be? Well, when children chronically withhold stool, a large, hard mass piles up in the rectum. But softer stool can still ooze around the hard lump and make its way out. When a child poops multiple times a day, this usually signifies the child isn’t fully emptying.

So, if a child poops out corn 24 hours after eating it, that tells you . . . nothing.

I learned this early in my career. At the time, I knew constipation was a major cause of wetting and UTIs, so I offered children the constipation therapies I’d been taught, primarily oral laxatives such as MiraLAX and, on occasion, Ex-Lax. But most of my patients didn’t improve, and the families would come back to my clinic frustrated. This troubled me deeply. I couldn’t figure out where I was going wrong.

Because I’m a urinary specialist, not a GI expert, I would often refer these patients to gastroenterologists — I figured my GI colleagues would have a better handle on how to diagnose and treat constipation. But the GI docs would send my patients right back.

“I don’t get it,” one GI doctor told me said. “These kids you send me—they aren’t constipated.”

She said my patients pooped every day and had normal marker studies. In other words, when they swallowed capsules containing special markers that show up on an X-ray — a fancier version of the corn test — the poop moved through the colon in a timely manner.

I scrutinized the X-ray of a child this GI doc had described as “not constipated” and saw a rectum stuffed with poop. That’s when it hit me: We have a definition problem.

The conventional understanding of constipation — infrequent pooping — didn’t fit. Though some stool was passing through on a daily basis, poop was nonetheless piling up in the rectum. These big, hardened clumps were affecting the bladder in serious ways.

OK, so if the corn test is worthless, what diagnostic methods are reliable?

Well, for definitive proof I recommend an abdominal X-ray, known as a KUB (for kidneys, ureters and bladder), which your pediatrician can order. (Are X-rays for constipation safe? Yes.)

But there are simpler ways of detecting a rectal clog. As you can see from our infographic 12 Signs a Child is Constipated, the biggest red flag is extra-large poops. If your 5-year-old’s poops make you go, “Whoa, Nelly!” or if they clog the toilet, well, your child is constipated.

The same applies if the child’s poops that are hard and formed, like a log or rabbit pellets. Match your child’s poops with the illustrations in our How’s Your Poop? chart, and you’ll pretty quickly know if your child is constipated. (And if so, treat the constipation aggressively!)

Any child who has poop accidents is severely constipated, and so are almost all children with daytime and/or nighttime wetting.

So, there are lots of ways to tell if there’s a poop pile-up in your child’s rectum.

The corn test just isn’t one of them.

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