At Auckland City Hospital, about 10 surgery patients in every 1,000 will suffer a major internal infection after their operation.
The rate has trended upwards since 2005, when it was about eight patients per 1,000. Other public hospitals have also had an increase.
The trends are revealed in data made public as part of efforts to make hospitals safer for patients.
Until now, the Health Quality and Safety Commission has released data on surgical infection rates and several other measures of healthcare quality only at the national level. Yesterday's publication of results breaks the figures down by district health board for the first time.
The Herald sought the figures from the commission last July. They were withheld on October 4 under the Official Information Act by commission chief executive Janice Wilson on the grounds they would be made public "soon" - in January, more than four months later.
The newspaper appealed to the Ombudsmen. Negotiations followed and the Herald was given an exclusive preview of the data.
The figures seem to show variations between DHBs, although the commission has explicitly avoided making such comparisons because of weaknesses in the statistics.
The commission's director of health quality evaluation, Richard Hamblin, said it was interested in infection rates and other adverse patient outcomes to show change over time related to good hygiene and other safety practices to prevent medical complications.
He discouraged inter-DHB comparisons because each board served a population whose underlying health risks varied.
This meant the infection rates and the other patient outcomes were not entirely under a DHB's control. Some DHBs also used varying definitions, which undermined comparisons.
"Our presentation of this data shall concentrate on demonstrating change and contextualising the process markers while minimising the capacity to generate inappropriate inter-DHB benchmarking," Mr Hamblin said in a letter explaining the system to DHBs.
There are three checks on preventive measures - hand hygiene, assessing risk of falling in the elderly, and use of a surgical safety checklist by operating room staff before and after surgery.
Each is linked to specific complications - staphylococcus aureus bloodstream infection for hand hygiene, fractured hip for falls checks, and two following surgery: blood clots and internal infection.
Mr Hamblin said there was good evidence each of the process markers was linked to the complications. "A number of studies show good implementation of the World Health Organisation's surgical safety checklist is associated with a reduction of around 30 per cent in a number of common surgical complications, two of which are sepsis (major internal infection) and deep vein thrombosis/pulmonary embolism (blood clots mainly in the legs or lungs)."
He said the national data showed DHBs were generally improving their compliance with the preventive measures, but with most of the complications or "outcome markers", "it's too early to tell. We should see the effects over the next year or so."
The exception, in a related programme, was the sustained drop in the number of intensive-care patients with infections linked to insertion of a deep internal catheter through a blood vessel. This follows the standardisation of processes for inserting the line.
Improved hand hygiene is linked to reduced rates of hospital-acquired infections, including those caused by the bacterium staphylococcus aureus. The marker for this complication of healthcare is the number of cases of staph "bacteraemia", in which bacteria are found in the blood, where they can lead to a potentially fatal infection.
Nationally, the rate of this condition has remained relatively unchanged in the past 18 months at around 0.15 cases per thousand inpatients.
In the first half of last year, Auckland rates were 0.26 at Auckland DHB, 0.08 at Counties and 0.02 at Waitemata. Waikato's rate was 0.5.
However, some DHBs appear to have under-counted their number of patients or "bed days", which artificially inflates their bacteraemia rate. "For some reason we're only reporting 40 per cent of our bed days," said Waikato's chief medical adviser, Dr Tom Watson.
Waikato came second-to-last in the latest audit of hand hygiene compliance, at 66 per cent, compared with 71 per cent nationally.
National hand hygiene chief Dr Joshua Freeman said Waikato's inclusion of lower-risk wards in checks on whether staff were cleansing their hands when they should "does appear to have reduced their overall compliance figures".
Waikato has begun fresh rounds of education to improve its compliance and has put up posters to empower patients to ask if their doctors and nurses have cleaned their hands. The DHB said despite the hand hygiene figures, Waikato Hospital patients had benefited greatly from the focus on avoiding cross contamination.
A fracture in the hip - in the neck of the thigh bone - is a serious injury, even more so for the elderly.
The commission is trying to bring down the number of these cases, which stands at just under 100 a year nationally, by monitoring hospitals for the rate at which they assess elderly patients for risk of falling.
Counties Manukau DHB came first equal on that marker last year, checking 97 per cent of elderly patients, and for all DHBs, the rate increased from 77 per cent to 87 per cent.
At Counties, 11 elderly patients suffered a fractured neck of femur in a fall in the last June year.
The number at Waitemata was 11, Auckland 2 and Waikato 4.
"You can get enormous variation between years without it meaning there has been a change in the approach or safety of your organisation," said Counties' chief medical officer, Dr Gloria Johnson.
"It tells us we do need to try keeping as good as we can at doing the risk assessments."
The DHB employed many ways of trying to reduce the chances of at-risk patients falling.
These included non-slip socks, hourly or more frequent nurse checks to see if the patient needed help to the toilet or more pain relief, low beds, walking frames, minimising use of sedating medication, and using soft-wrap hip protectors for those at high risk of hip fracture.
A watch was being kept on foreign studies of whether timber was a safer option for a flooring base than the increasingly common concrete.
This is a potentially fatal infection of the blood and organs. Nationally, the commission reports there were 604 cases in the 12 months to last June, up from 531 cases in the 12 months to December 2012.
At Auckland DHB, the number of cases per 1,000 surgical patients _ the data excludes some operations such as for cancer and childbirth _ has risen from around eight in 2005 to more than 10 in the middle of last year. Northland's rate has averaged 14 over the period, varying from 8 to 18. Waitemata's averaged around 7.5 and Counties Manukau's just under 9 and Waikato's 7.6.
Waikato had a run of seven quarters above average to the middle of last year, which the commission suggested was getting close to a statistically significant rise.
The Auckland DHB's clinical adviser on quality and safety, Dr Colin McArthur, said his DHB's relatively high rate was due to the greater complexity of cases it treated.
"Most hospitals have seen a rise in perioperative sepsis over the last few years. We think the reason for that is that over the last five or six years we are increasingly able to offer interventions to patients with more significant co-morbidities (illnesses) or pre-operative complexity.''
Dr McArthur criticised the way in which appropriate use of the surgical safety checklist was measured.
Auckland DHB theatre staff were high users of the checklist before surgery, but nurses often wrote nothing on the section of the sign-out form for raising issues with post- operative staff _ leading auditors to conclude, wrongly, that the form had not been used. The commission plans to add direct observation of theatre staff to the auditing system.
Post-operative blood clots
A blood clot or thrombosis can form in the deep veins of the legs - a DVT. A piece, an embolus, can break off and move to the pulmonary vessels of the lungs. DVTs are painful, cause breathlessness and are slow to cure. A pulmonary embolism can kill. Risk factors for developing a clot include prolonged immobility, major surgery, previous clots, obesity, smoking and taking some types of oral contraceptives.
Nationally the annual tally of post-operative DVT and pulmonary embolisms was 712 to last June, down from 759 as at December 2012.
Waitemata DHB's 2005-2013 average was 7 cases in 1,000 surgical patients. Auckland's was 5.6, Counties' 3.5 and Waikato's 4.5.
The commission, relying on statistical analysis, said there was no clear trend at Waitemata.
However, the Waitemata clinical leader for quality, Dr Penny Andrew, said there had been a downward track since 2010.
"It now averages 6 and is heading in the right direction.
"We have a very proactive haematology department. They track every DVT. We analyse every case to see whether they receive the right treatment and prophylaxis (preventive therapies).
"We've got an improvement project drilling down on that data and working with our orthopaedic - hip and knee replacement - surgeons, making sure that they keep coming down."