Sheffield's Northern General Hospital is facing legal action after the women were wrongly told that their babies had a low risk of developing Down's syndrome.
The error remained undetected for nearly five months, although midwives had raised concerns. Laboratory staff were confident that the hospital's Pathlan computer had been properly checked for Y2K compliance.
But the results of an inquiry published last week revealed that both the pathology laboratory computer services manager and the external support engineer responsible for a Pathlan upgrade did not have the IT skills to devise the crucial Y2K tests.
Like many hospital laboratory staff, they developed an interest in IT out of necessity as computers were introduced into the NHS.
The laboratory's computer services manager had taught himself Basic programming on a BBC computer, the inquiry found.
The Sheffield laboratory had relied on Y2K testing by Hartlepool and Peter Lee Hospitals Trust, which had taken over the maintenance contract for Pathlan from ICL.
But while Hartlepool had fixed known Y2K bugs in the code, the Sheffield laboratory had been unaware that Hartlepool lacked the time and resources to check other parts of PathLan, including Sheffield's Down's screening software.
Further Y2K tests by the laboratory's computer systems manager on site "were inadequate to detect the fault", which meant that the ages of some women were wrongly calculated as negative numbers.
The original software was poorly written - even by the standards of the time - the report concluded. It lacked internal error-trapping that could have identified the problem at an early stage.
The failures were compounded by a poor user interface, which meant that the intermediate calculations which would have alerted staff immediately were not displayed.
The inquiry found that technical shortcomings were exacerbated by a series of managerial mistakes.
Laboratory staff did not have access to support from the hospital's IT department and Pathlan fell outside the hospital's main Y2K program. There was no line management responsibility for ensuring the upgrade had been completed successfully.
The laboratory was reliant on the skills of one technician at Hartlepool Hospital with expert knowledge about the system. There were no contingency plans for coping if he was not available.
Crucially, the hospital had been alerted to the need to improve its IT procedures by the discovery of another programming error in 1994, which led to risks of Down's syndrome being miscalculated if the weight of the patient was unknown.
The hospital then gave assurances that it would introduce validation checks and improve software change controls. This was no more than good practice, the report said, but Hartlepool Hospital, which maintained the system, was not told.
The laboratory's lack of IT skills could also have been addressed if the hospital had complied with the recommendations of a later internal audit report that raised concerns about the lack of technical expertise in the immunology department.
The inquiry has raised wider concerns that labs responsible for analysing patient tests elsewhere in the NHS may be using in-house staff for first-line IT support, rather than qualified external IT staff.
Both hospitals involved said they would act on the inquiry's findings.
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