Authors (including presenting author) :
Chu KY (1), Leung WYS (1), Chan TF (1), Cheung MY (1), Law LT (1), Leung MH (2)
Affiliation :
(1) Pharmacy Department, Queen Elizabeth Hospital, (2) Department of Medicine, Queen Elizabeth Hospital
Keyword 1: :
Ishihara test
Keyword 3: :
Pharmacist Anticoagulation Clinic
Keyword 5: :
colour vision deficiency
Introduction :
Warfarin is an anticoagulant that is commonly prescribed for the treatment and prevention of thromboembolic events. Due to its low therapeutic index, subtherapeutic and supratherapeutic levels can increase the risk of thromboembolic and bleeding complications respectively. Colour code is often used among healthcare providers and patients in dose titration of warfarin. Two strengths of warfarin with different colours are available in Hong Kong. Previous study showed that 8% of men and 0.4% of women have impaired colour vision which may affect their recognition of tablets including high-risk medication warfarin.
Objectives :
To investigate the prevalence of male patients taking warfarin with colour vision deficiency, evaluate their drug administration habit and any manifest medication incident.
Methodology :
Ishihara test was applied to all male patients from April 2023 to March 2024 in the Pharmacist Anticoagulation Clinic in Queen Elizabeth Hospital.
Result & Outcome :
There were 212 male patients completed Ishihara test. Eighteen patients(8.5%) failed in the test. The prevalence is consistent with findings in previous literature. Upon pharmacist assessment, only fourteen patients can differentiate warfarin tablets while four patients(1.89%) failed to distinguish the colour of two warfarin strengths. In these four patients, the regimen was changed to one single strength of warfarin. One patient required assistance from caregiver when administered warfarin, two patients differentiated the tablets by the numerical markings on them or drug labels. One patient described brown warfarin 1mg tablet as purple, blue warfarin 3mg tablet as green. None of the patient reported any medication incident in administration of warfarin. This is the first study to investigate the association between colour blindness and medication safety in warfarin in clinical setting. There are over 13,000 patients on warfarin under the care of Hospital Authority. In this study, there are 1.89% male patients unable to differentiate the colour of warfarin tablet. As tele-health has rapidly evolved in practice, healthcare providers should ensure patients can recognize tablet colour in communicating drug dosage. If patients are suspected of impaired colour vision, doctors and pharmacists should prescribe one strength of warfarin to avoid mix-up. Proper communication in the healthcare team should be done. Manufacturers are advised to use blister packaging for warfarin. Healthcare providers should be aware of potential medication risks in patients with colour blindness, particularly when drugs are differentiated with colour including warfarin, insulins and inhalers.
