Credibility is a vital component of medical screening services. Without it, they stand to lose the confidence of the subjects for whose benefit they exist.
Shown to be ineffective or unreliable, such socially beneficial preventive programmes leach support and patronage.
This can lead to tragic and unnecessary loss of life. It is too early to tell whether there is a serious prospect of this occurring as a result of the latest revelations on the Southern District Health Board's breast screening programme, but that shortcomings have been identified is serious enough in itself.
Those revelations, arising out of an internal audit, concern possible delays in diagnosing breast cancer in 28 women over a three-year period. The problem relates to the reading of mammograms at BreastScreen HealthCare, the service for Otago and Southland women.
This is the third time in its relatively short history - the programme in Otago began with a pilot in 1991 ahead of a national screening programme in 1998 - that performance issues have compromised the service.
In 2000, the screening programme, as it was then, invited controversy when it was discovered that the mammography files of up to 60 women were found to contain errors, and two women developed malignant breast cancer. The errors were of an essentially clerical nature relating to the transfer of data from screening records to computer.
In January this year, it emerged that computer software problems in 2009 led to 241 women in the lower North Island and the Southern DHB area missing out on their two-yearly mammogram appointments.
Five of these women were subsequently found to have cancer. While no consolation to those who missed their appointments and who were subsequently discovered to have developed cancer, software errors are generally able to be rectified once they are discovered. Immediate remedies are less apparent when the mistakes or omissions are down to human error and questionable expertise or procedures in the reading of mammograms.
The initial indications are that this may be what has instigated an urgent investigation, including a review of the service, by external senior radiologists. And while the Southern District Health Board says it follows best practice in having each mammogram seen by two practitioners, that two readings in each case may have failed to detect early warning signs could be seen as further cause for concern.
The potential diagnosis delays were found in a clinical audit of screens taken between 2007 and 2010 of women who later developed breast cancer.
A DHB radiologist carried out the audit because of concerns about the low rate of small cancer - which usually means early cancer - diagnosis at the Southern DHB.
While an extremely serious departure from the high standards the Southern District Health Board would want to set itself, it needs to be seen in context.
The service screens about 16,000 women each year, which is getting on for 50,000 over a three-year period. This amounts to a failure rate of about 0.05%, but with potentially fatal consequences, even one missed diagnosis is one too many.
Pending the outcome of the investigation, there is now a possibility that thousands of women might have to be rescreened. In the meantime, it has emerged that BreastScreen HealthCare has lost a full-time radiologist to Christchurch and the service is understaffed. Counties Manukau DHB and MidCentral DHB radiologists are providing second reads of mammograms and an additional two radiologists are heading to the SDHB to perform assessment clinics (when a woman is recalled after a mammogram) indefinitely.
"Basically what we're looking at is providing some additional and new eyes around the process that we're going through," explained Ministry of Health chief medical officer Don Mackie yesterday.
Those new eyes will be especially welcome - for the women in the breast-screening queue today, the next day and the day after, but also for those who face the prospect of being reassessed. They have every right to expect the procedures deployed at the Southern DHB are of the highest standards of rigour and accuracy. Unfortunately, the symptoms suggest this may not necessarily be the case.
