My colleague, Dr. L.T. Kim, was off this week and I covered for him.

On Friday afternoon, I dealt with two of his patients and learned — or relearned — two important lessons.

I saw a man with thoracolumbar back pain. He had fallen off a ladder a few years earlier and suffered from recurring bouts of back pain — sometimes with tingling in both legs. He had been to the emergency room after a particularly bad episode. Dr. Kim saw him in a follow-up and ordered an MRI of his thoracic spine.

I saw him to review the results. The MRI showed more or less garden-variety degenerative changes, but nothing that would explain all his symptoms.

“I’m feeling much better, but this very sore spot is still here,” he said and asked if he could point to the corresponding place on my back.

I asked him to remove his shirt and palpated my way down his spine.

“Right there. You got it,” he said.

I marked the spot with an X with my green ink rollerball pen, sat down at the computer and ordered PA and lateral lumbar spine films. My tech taped a metallic marker over my X, and a few minutes later I saw on the screen that his pain centered on his second lumbar vertebra, just below where his expensive MRI had ended.

A call to Cityside Hospital’s MRI department verified that they couldn’t just go back and look a little lower on their images, which only included a small fraction of L2. Our patient needed a whole new, lumbar MRI.

In case I had any temptation to feel a little smug that I had realized something Dr. Kim hadn’t, I learned another lesson at 4:55 p.m.

“I’ve got a sodium of 123 on one of Dr. Kim’s patients,” our lab manager said as she entered my office with a lab printout in her hand. “If he saw this he’d probably have the patient go to the ER by ambulance,” she continued.

“Well I don’t usually worry quite that much about sodium levels,” I said. “I’ll take care of it.”

I saw that this older woman had been discharged from the hospital a week earlier, and she ran low sodium there — about 130.

Dr. Kim is an internist by training, and he spent most of his residency years in a tertiary acute care hospital, where only the sickest patients went. In that setting, even small changes in lab values could be harbingers of deterioration, disaster and death. I spent most of my training in small-town hospitals and outpatient clinics, where most people got better more or less on their own, and where small laboratory abnormalities often didn’t matter much at all.

I dialed the number.

“Hello, is this Mrs. Weld? This is Dr. D. calling from the clinic with your lab results. Dr. Kim is away this week.”

“No, this is her daughter.”

“Her sodium is low, so I’m calling to see how she is doing.”

There were several voices in the background.

“Guys, I’ve got the doctor on the phone,” she said, and the voices went silent. She continued, “The ambulance is here, I’ll put you on speakerphone so you can talk with them.”

“Hey, Doc, what’s up?” the familiar voice of one of our local EMTs greeted me.

“Mrs. Weld has a sodium of 123, it was 130 a week ago when she left the hospital”, I said.

“What are the symptoms of that?”

“Weakness, lethargy, confusion … ” I started.

“That would be it, Doc.”

“So she needs to go back to the hospital. I’ll call the ER”, I said.

“Thanks a lot for calling, Doc. Good timing!”

Indeed. And I thought this would turn out to be just an insignificant laboratory abnormality.

“A Country Doctor” is a family physician who blogs at A Country Doctor Writes:.

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