Hospitals in England choose which patients to treat based on specific goals rather than clinical need, the researchers suggested.
Those behind the study warned that referring to 18 weeks as the treatment standard gives hospitals “no incentive” to treat those who have only waited a short time or who have passed the limit.
The target – set at 92% – was introduced in 2012 to ensure patients do not wait more than 18 weeks from GP referral to starting treatment for routine, non-urgent conditions.
But a team spanning the University of Birmingham, University Hospitals Birmingham NHS Foundation Trust and the West Midlands Academic Health Science Network said it could have “unintended consequences”.
The researchers refer to this as the “threshold effect,” which appears as a spike in the data, known as a discontinuity, that occurs around the target boundary.
They analyzed monthly waiting time data for all 144 non-specialist NHS hospitals in England between January 2016 and September 2021.
The group said the data “repeatedly showed a large increase in the number of trusts meeting exactly the 92% target threshold for the 18-week standard, followed by a sharp drop after the target was met”, suggesting that some trusts are facing the minimum number of patients waiting under 18 weeks to meet the standard.
They added that “the target provides no incentive to treat patients who have only been waiting a short time or who have already passed the 18-week wait.”
“Policy makers should be careful in their use of targets. Second, if targets are used, then policy makers should regularly check for threshold effects. Third, targets should be carefully designed to mitigate threshold effects.”
The figures revealed that the number of NHS hospitals meeting the target fell from 92% in 2015/16 to 64% in 2021/22.
The proportion of trusts where patients waited less than 18 weeks also fell after a financial penalty for missing targets was lifted in 2016/17.
Nigel Edwards, chief executive of the Nuffield Trust, also said the study – published in the journal BMJ Quality & Safety – “joins a long litany of examples of the unintended effect of targets”.
He added: “The most sustainable approach to achieving a goal is to redesign processes and realign resources to ensure that goals are achieved as a by-product of a well-designed system.
“However, if there are insufficient resources – for example, in the case of waiting lists where demand exceeds capacity or the organization lacks the skills and resources to undertake a major review of processes and ways of working – less desirable approaches. “
Mr Edwards said these could include demanding an “unsustainable” level of work from staff.
He added: “The experience of using targets in the English NHS, as demonstrated by (the study authors) and many other researchers, suggests that over-reliance on a small number of high-profile measures is dangerous.
“A richer picture is needed of how the system being measured works and how its staff and managers behave and are motivated for sustainable long-term change.
“Effective performance improvement systems cannot be built solely on objectives, but require great management judgment.
“The improvement system and the local teams that deliver improvement both need the managerial ability to make good judgments to avoid the issues of gaming, of over-promising ways of pursuing measurement without missing the point.”
An NHS spokesman said: “The suggestion that the NHS is incentivizing trusts not to bother people when they are past 18 weeks is nonsense – the priority of treatment is determined by the clinical needs of patients as determined by clinicians as well as those who they wait longer.
“The NHS has a clear plan in place to reduce the Covid backlog that has inevitably built up as a result of the pandemic, with significant progress reducing the longest waits.”
The concept of referral to treatment was first introduced to the NHS in 2004 – under the Labor government – with the 92% target introduced 12 years later.
The Labor Party and the Department of Health and Social Care have been approached for comment.