One in five bowel cancer patients diagnosed in an emergency had "red flag" symptoms that should have been picked up earlier, a study in the British Journal of Cancer suggests.
And 16% of emergency bowel cancer patients had seen their GP three times or more with relevant symptoms.
Overall, these symptoms were less common in patients diagnosed at an emergency stage rather than earlier on.
The study looked at data on diagnoses in England from 2005 to 2006.
The researchers, from University College London and the London School of Hygiene and Tropical Medicine, focused on what had happened to patients in the five years leading up to their cancer diagnosis.
They analysed National Cancer Registry data that was linked to GP data for 1,606 patients from more than 200 GP practices.
They found that in England 35% of colon cancers and 15% of rectal cancers were diagnosed after an emergency presentation.
Although many of these patients often did not display obvious symptoms for bowel cancer, making it harder for GPs to diagnose their cancer early, 17.5% of colon cancer patients and 23% of rectal cancer patients did have "red flag" symptoms.
These symptoms include rectal bleeding or blood in the stools, a change in bowel habit and stomach pain.
The researchers say these figures indicate there could have been opportunities to pick up the disease earlier.
Paul, 47, from Manchester, started to feel unwell in 2007. He had no energy and constantly felt worn out.
Something was wrong, but the GP told him to lose some weight and come back if things did not improve.
He did go back - several times - and eventually, after he noticed blood in his stools, his GP arranged for a sample to be sent away for tests.
But there was a delay after his name was not put on his sample. And then, the test came back saying everything was OK.
However, he was increasingly concerned at feeling tired all the time. Sometimes after work, as a self-employed builder, he had to go straight to bed.
Finally, early in 2008, with the encouragement of his family and friends, he decided to pay to see a specialist privately.
Within days of having an internal examination and a scan, he was diagnosed with bowel cancer that had spread to his liver and lungs.
The following week, Paul had the first of several major operations to remove the cancer.
He says: "I was practically begging them in the end to do something.
"They told me that if I'd left it another couple of months, the cancer would have spread and I would have had no chance."
Doctors removed more than 2ft (60cm) of his bowel before taking half of his liver out too, then chemotherapy followed.
"My energy levels are back to a certain extent, but I'll never be the same as I was," he says.
"I can't do all the things I used to do because of all the surgery I've had - but I count my blessings."
Most patients, regardless of how they were diagnosed, visited their doctor in the year before their diagnosis.
Patients tended to see their doctors more often in the months and days leading up to their cancer diagnosis - whether it happened in an emergency or not.
The research team suggested that specially trained nurses could support GPs and act as a safety net during consultations.
Easier access to specialist advice for GPs was also important, the study said.
Cristina Renzi, lead researcher from University College London, said patients diagnosed with cancer after an emergency presentation did not fare as well as patients diagnosed by their GP.
"However, in most cases they visit their doctor for various reasons multiple times during the months leading up to their diagnosis, which could represent opportunities to diagnose the cancer earlier.
She added: "It's important to find ways to ensure these patients can be diagnosed at an early stage.
"And this study highlights the need to support GPs and give them the tools to diagnose and refer patients promptly when they feel it's necessary."
Dr Julie Sharp, head of patient information and health at Cancer Research UK, said diagnosing patients who were not showing typical symptoms of bowel cancer was very difficult.
"So research like this, to understand more about people's symptom histories, is crucial to find better ways to diagnose the disease at an early stage, when treatment is more likely to be successful."
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