But both sectors admit it is difficult to control such procedures because doctors can always claim medical necessity.

Doctors say they need to practice 'defensive medicine' amid growing patient-rights activism. They also point to increasing expectations among patients as a factor leading to over-investigations.

Patients complain of having to foot bills for tens of thousands of dollars for mild illnesses and unnecessary tests. One patient, a school teacher, said he felt angry after being admitted to the intensive care unit at a private hospital with mild flu symptoms. The 46-year-old went to the outpatient clinic on December 27.

'I had a cough, headache and runny nose,' he said. 'A doctor at the private hospital asked me to have a heart scan and said I might have suffered a myocardial infarction [heart attack]. He said I needed intensive care. I and my wife were so shocked and did not know how to react.'

He asked to be discharged the next day as his all vital signs were normal.

'I took my medical reports to two other doctors and both said there was no need for that doctor to have put me in intensive care. I felt cheated. The doctor just used me as a tool to make money.'

After paying his HK$21,000 bill he filed a complaint with the hospital.

Patients' Rights Association spokesman Tim Pang Hung-cheong said the group was waiting for the findings of the hospital investigation in this case. He said patients should ask more about their treatment and tests.

'They should ask more questions and for explanations on why a certain kind of test is needed,' he said.

Doctors and insurance companies are often on opposite sides on this issue. While doctors accuse insurance companies of interfering with their clinical decisions, the companies say they need to play a gatekeeping role to control medical costs.

A senior insurance company executive said that six or seven years ago, there were only four to five hospital admission claims each year per 100 medical insurance policies. That has risen to six to eight per 100 policies. Medical insurance premiums increase by an average of 3 to 5 per cent a year to offset the cost.

'We understand that some increases in claims are the result of medical advances and patients' expectations. But we are also concerned if unnecessary procedures are boosting the number.'

The executive said the phenomenon of over-investigations had become more widespread in the past two or three years.

The executive's company received a claim from a middle-aged woman who underwent five different investigations in one day.

'Conducting an excessive number of procedures is not a good practice, unless it is absolutely necessary. You can also imagine how uncomfortable and painful the patient may feel,' the executive said.

On another occasion the insurance company raised questions after a patient was seen by six doctors at the same time.

'What bothered us was that the general practitioner continued to see the patient together with the five other specialists so he could charge his fees. We didn't find his presence necessary,' the executive said.

Some doctors claim an anaesthetics fee although they are not registered anaesthetists. They might, for instance, claim for injecting a patient with a sedative before a colonoscopy.

The insurance executive said individual companies now kept their own watch list of doctors.

'Our company, for example, keeps a close eye on several doctors who make unreasonable claims.'

Some companies have referred cases to the Medical Council and the Independent Commission Against Corruption. But they have not received any positive responses.

Individual firms are considering their own controls, such as limiting the number of doctors who can see a patient each day and the number of procedures done per day.

'We are exploring various options in designing our new products. But we understand that it is rather difficult to limit the number of tests done per day. Tightening the coverage and benefits remains the most effective way to curb abuse,' the insurance executive said.

Peter Tam Chung-ho, executive director of the Hong Kong Federation of Insurers, said some insurers had raised concerns about over-investigations, and the federation provided a platform for sharing information.

'The phenomenon is happening among a small group of doctors. Most doctors behave and we always respect doctors' professional autonomy,' Tam said.

The chairwoman of the Medical Council, Professor Felice Lieh Mak, said it was difficult to draw a line between necessary and unnecessary procedures.

'If the Medical Council receives a complaint, it will look at that individual case's evidence. It is very difficult to prove a procedure was unnecessary because some doctors are particularly careful and delicate in making a diagnosis.'

Lieh Mak said over-investigations were a product of 'defensive medicine'. 'It is an American culture where doctors make sure they have done every possible step to avoid being sued for negligence. It is difficult to draw a line between due diligence and overdoing. Sometimes doctors give in to patients' demands and expectations for more and more tests.'

Lieh Mak said doctors nowadays relied on advanced technology in diagnosis such as magnetic resonance imaging, computed tomography scans and radiography.

One patient said she was pressured by a private doctor into undergoing tests she did not want. The middle-aged woman was referred by her insurance agent to the private doctor for a heart scan in June 2008.

'Once I was at the hospital, the doctor kept asking me to have other tests on my intestines and stomach. I did not understand what the tests were about and said I did not want to do them. But my insurance agent kept pressuring me, saying my insurance policy covered the costs, I finally agreed to have the tests although I was not willing.'

She ended up with a bill for HK$70,000, but the insurance company only reimbursed HK$40,000.

Dr Choi Kin, a former president of the Hong Kong Medical Association, said over-investigations were not necessarily the same as professional misconduct.

'Some investigations are requested by patients and some inexperienced doctors also rely on more tests to come up with a diagnosis, but it is not misconduct.'

Choi said that as medicine became increasingly specialised, a patient may need more than one doctor to take care of him.

He recently treated a patient who had multiple problems including heart disease, colon cancer, diabetes, hypertension and asthma.

The patient needs a heart operation and, according to guidelines by the College of Cardiologists, he needs two cardiologists - one for the operation and another on standby for emergencies.

While Choi takes care of this patient's chronic diseases, the patient also needs a colon surgeon to check his colon cancer.

'In this case, the patient needs a total of four doctors during his hospital stay,' Choi said.

'Specialisation is an irreversible trend. Medicine will only become more and more expensive because doctors tend to specialise in one field only and they need help from their peers if a patient has more than one medical condition.'

Overcharging is also a concern although insurers admit it is difficult to measure. In one case a patient was charged more than HK$130,000 for a circumcision at a private hospital. According to the latest survey of doctors' fees by the Hong Kong Medical Association, the median charge for a circumcision is HK$7,000.

The insurance executive admits that it is difficult to define what is 'overcharging' in a free market.

'Although we spot excessive charging, we can't do anything about it. What we can do is to reimburse according to the plan's benefit.'

The president of the Academy of Medicine, Professor Raymond Liang Hin-suen, said individual colleges could not stipulate treatment guidelines for all kinds of conditions. The academy trains specialists.

Liang said some procedures were requested by patients. 'Some doctors want to be careful in their diagnosis so they need various kinds of investigations, it is very difficult to judge if an investigation is necessary,' he said.