The MHRA said last week there had been 'no reported adverse incidents' but it was due to issue a medical device alert.
SmartCheck meters were used by GPs providing near-patient testing for patients on anticoagulation therapy.
In the post-launch study the mobile phone-sized monitors 'did not produce equivalent performance to the claims in the product labelling', said Unipath.
Patients' INR of blood clotting could be wrongly estimated, putting them at risk of undermedication, and the possibility of recurrent thrombosis or embolism, or overmedication with the risk of spontaneous haemorrhage.
Up to 100 surgeries that have been using SmartCheck meters will be invited to switch to the Hemosense INRatio system.
Dr Chris Gardiner, chief biomedical scientist in the haematology department at University College Hospital, London, who evaluated the device for the NHS before its launch in April 2007, was surprised by the recall.
'We were quite happy with it,' he said.
GPs incentivised to take on primary care based anticoagulation are worried they could be liable if a faulty reading led to adverse consequences.
But Dr Nick Clements, senior medicolegal adviser at the Medical Protection Society, said GPs unaware of problems with their SmartCheck monitors should be protected, so long as they used it in accordance with the manufacturers' recommendations.
GPs should report any device problems to the manufacturer, he added.
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