People who wait up to nine months to get a colonoscopy after fecal tests show potential tumors are no more likely to be diagnosed with cancer or advanced malignancies than those who get follow-up colonoscopies sooner, a U.S. study suggests.

Colonoscopy is the most commonly used screening test for colorectal cancers, but it’s invasive, costly and inconvenient. A growing number of patients are instead using screening kits known as fecal immunochemical tests (FIT) that can detect tiny amounts of blood in the stool that might indicate cancer. Positive FIT results require a follow-up colonoscopy to find any tumors and assess their severity.

For the study, researchers examined data on how long more than 70,000 patients with positive FIT results waited to get follow-up colonoscopies. When they waited no more than one month, 30 cases out of every 1,000 colonoscopies confirmed cancer and 8 cases per 1,000 showed advanced tumors. Results were similar for patients who waited up to nine months.

But when patients got colonoscopies 10 to 12 months after positive FIT results, there were 49 cases of cancer confirmed for every 1,000 colonoscopies, or 48 percent higher odds than waiting no more than one month. The odds of advanced cancer were roughly doubled, with 19 cases per 1,000 colonoscopies.

“Colonoscopy can be a complicated test to schedule, requiring time off from work and someone to accompany the patient home,” said lead study author Dr. Douglas Corley of Kaiser Permanente Division of Research in Oakland, California.

“This study suggests patients have a little time to make arrangements for the exam around their work or personal schedules,” Corley said by email. “These data confirm that colorectal cancers progress fairly slowly and that it is feasible to get large numbers of patients in for follow-up fairly rapidly.”

Half of the patients in the study had colonoscopies within 37 days of a positive FIT result, the researchers report in JAMA.

Almost two-thirds of patients with positive FIT results got follow-up colonoscopies within two months, and 81 percent of them had a colonoscopy within six months, the study found.

Patients ranged in age from 50 to 75 years old, and half of them were at least 61.

Overall, there were 2,191 cases of any colorectal cancer and 601 cases of advanced disease diagnosed.

Compared to people who got colonoscopies within a month, patients who waited a year or more had more than triple the odds of having advanced malignancies at the time of diagnosis.

The study isn’t a randomized experiment designed to prove whether or how the time between FIT results and a colonoscopy directly influences the risk of cancer or of advanced tumors, the authors note. It’s also possible the study might not have detected differences between patients who got colonoscopies sooner and people who waited longer for follow-up tests.

“It could be that people who return for colonoscopy several months after a positive screening test are returning because they have symptoms and so are more likely to have cancer detected,” said Carolyn Rutter, a senior statistician at RAND Corporation in Santa Monica, California, and co-author of an accompanying editorial.

Symptoms can include diarrhea, constipation, rectal bleeding, cramping, fatigue or weight loss - all of which could also be due to conditions other than cancer.

“Fecal-based tests are a very effective, inexpensive, and non-invasive approach to screening for colorectal cancer,” Rutter said by email. “But they can only detect cancer when positive tests are followed by colonoscopy, and because they are most sensitive for detection of preclinical cancer, fecal-based tests need to be repeated annually.”

While it’s not ok to skip that follow-up colonoscopy, the study results suggest it’s fine for patients to wait a little bit to schedule the exam at a time that works best with everything else they have going on in their lives, Rutter said.