Most quit attempts self-reported by patients as ‘successful’ are not being confirmed by a blood test, NHS Information Centre data show.
Experts said the lack of validated data meant more patients may relapse than current figures suggest.
GPs warned that checking must improve and said patients should be followed up for longer to improve quit rates.
Smokers’ quit attempts are deemed successful if they self-report as having not smoked for four weeks after a set quit date. This should be confirmed by measuring carbon monoxide in their bloodstream.
These data are used to estimate the likely number still not smoking one year after joining the programme.
But from April to December 2010, the NHS tested just 34% of the 245,664 patients who self-reported as having successfully quit.
Tests showed almost 75,000 patients – one in three – who reported they had quit had in fact relapsed within this four-week follow up.
Birmingham smoking cessation expert and GP Professor Paul Aveyard said NHS quit data was ‘useless’ if services failed to validate a successful quit attempt.
He said: ‘It is concerning that some services rely too much on unvalidated and "casual" follow up at four weeks. Only with rigorous follow up and biochemical validation, can we estimate the number of long term abstainers.’
South west London smoking cessation GPSI Dr Alex Bobak said most clinicians ‘feel quit attempts should be followed up for 12 weeks’.
A GP investigation (26 May) found 33% of PCTs slashed funding for smoking cessation services by up to 90% in 2010/11.