NHS Greater Glasgow and Clyde (NHS GGC) launched an inquiry in November last year after fertilisation rates at Glasgow Royal Infirmary's Assisted Conception Unit (ACU) plunged from about 30% to close to 15% between September 1 and November 9. It was thought air pollutants from building works on the floor above may have been to blame.
Patients using the Royal's service were subsequently transferred to the city's private Nuffield hospital, with women who had undergone IVF during the period under investigation offered a complimentary treatment cycle. The Nuffield is continuing to cater for around 25 NHS patients per week with the service not expected to return to the Royal until the end of this year.
However, the hospital's own in-house investigation has now been wound up with mystery remaining as to what caused the dip in fertility rates – although clinical error has apparently been ruled out.
A report has been passed by the health board to the UK regulator, the Human Embryology and Fertility Authority (HEFA), which is expected to respond to the findings in the next few weeks.
Meanwhile, NHS GGC has appealed to other private fertility clinics to tender for contracts to take on some of the NHS patients who will be unable to use the Royal, to ease the burden on the Nuffield hospital.
It comes as work begins on the £1 million refurbishment of the ACU, including the fertility laboratories at the centre of the investigation.
A spokeswoman for NHS GGC said: "A clinical review was commissioned, led by the clinical director responsible for the Assisted Conception Service, which looked at clinical processes within the ACS unit and other broader environmental factors that may have impacted on success rates.
"The review was unable to establish conclusively the exact nature of the cause of the drop in success rates although it does appear the decrease in success rates was not as a result of clinical practices.
"As such we have decided that in the medium term the Assisted Conception Service should continue to be provided off site in the private sector while a significant £1m refurbishment of the unit, including laboratory facilities, is carried out.
"The refurbishment is already under way and we expect a full return of the service to the Glasgow Royal Infirmary by the end of 2013. During this time we have been in open dialogue with HEFA and it has been informed as to the outcome of the clinical review."
Professor Richard Fleming, director of the private Glasgow Centre for Reproductive Medicine, said it was not surprising investigators were unable to get to the root of the problem.
Mr Fleming said: "I'm certainly not surprised because even if you find something interfering with the survival of these [embryo] cells, how are you going to test them?
"I imagine that when they return to the site they will bring in better measures for air cleaning which will go some way to providing reassurance to patients."
It had been thought building works on the floor immediately above the unit may have disrupted the process, which is extremely vulnerable to contamination. Tiny amounts of dust, paint fumes and chemical vapours entering laboratories via ventilation shafts are all known to pose a potential danger to embryos being prepared for implantation.
The GRI is used by women and couples from NHSGGC, NHS Lanarkshire, NHS Ayrshire and Arran and NHS Dumfries and Galloway.