A raft of tests had failed to uncover the cause of rectal bleeding in a patient who was later operated on by Jayant Patel, a court has heard.

Mervyn John Morris died of complications, including heart and lung failure and septicemia, after Patel removed part of his colon at the Bundaberg Base Hospital in 2003, the Supreme Court in Brisbane has heard.

The crown alleges the operation should not have been done because of Mr Morris's medical history, which included heart problems and possible liver cancer.

Patel has pleaded not guilty to his manslaughter.

Giving evidence in the Supreme Court in Brisbane, Dr Marthinus Nel said he had concerns in early 2003 that abnormal liver functions meant Mr Morris might have a malignant cancer.

He said he referred Mr Morris to the Bundaberg Base Hospital, where an ultrasound report identified a "very suspicious mass".

However, the court was told subsequent tests showed normal results for two tumour markers.

Dr Nel said he remained concerned about the rectal bleeding, which he did not believe was from radiation proctitis.

Mr Morris had previously received radiation treatment for prostate cancer.

The court heard another doctor suggested Mr Morris may have been suffering from diverticular disease affecting the sigmoid colon.

Dr Nel agreed that was possible.

The trial continues.