A question mark remains over the quality of service provided by private clinics seeing NHS patients in England, the health watchdog has said.
Cataract surgery is one of the operations carried out by ISTCs
The Healthcare Commission said it could not assess independent sector treatment centres because of a lack of data.
It said there were reassuring signs but called on ministers to rectify the "cock-up" which meant performance data was not comparable with the NHS.
The government said the clinics, which perform minor surgery, were doing well.
Independent sector treatment centres (ISTCs) - where private companies have contracts with the NHS to run clinics - were introduced by the government in a bid to reduce hospital waiting times.
The NHS watchdog looked at the 23 ISTCs that formed the first wave of clinics in 2003.
So far they have seen over 600,000 patients needing relatively minor treatments, such as hip and knee operations and cataract surgery.
ISTCs have proved controversial with doctors accusing them of cherry-picking the easiest patients, depriving the NHS of money and medics of valuable experience.
The Healthcare Commission carried out the review into the quality of care provided by ISTCs after MPs expressed concern about their benefit.
Officials proceeded to interview 2,000 patients, analyse data supplied by the centres and carry out their own inspections.
But the watchdog has now announced it was unable to give a full seal of approval to the clinics because of gaps in data.
Healthcare Commission chief executive Anna Walker said there had been a "cock-up" in the way they had been set up after being introduced quickly.
The clinics are measured on a series of key performance indicators measuring patient outcomes which do not correspond with how the NHS collects data.
The watchdog also highlighted gaps in evidence on activities ISTCs were contractually obliged to provide.
Ms Walker said: "This must be rectified as we go forward."
Performance data must be collected by one body and be measured in the same way, she added.
The report also revealed that, in some of the areas the watchdog was able to assess, ISTCs were doing well.
It said emergency readmission rates were lower than in the NHS - although this was to be expected because ISTCs take on easier cases.
The patient survey also showed high satisfaction rates.
The British Medical Association's consultants committee said it was "dismayed to hear that there are still problems with data collection".
But Health Minister Ben Bradshaw said the report showed that ISTCs were "performing well in a number of areas" and that the government was looking to standardise performance data.
He added: "We remain committed to ensuring that patients can continue to expect high standards of quality in both NHS and independent sector."
The government is also funding the Royal College of Surgeons to carry out a three-year review to assess the success of the clinics.