Background: Warfarin is an anticoagulant that is used for the prophylactic and therapeutic
treatment for a wide range of thrombo-embolic disorders. The prescribing and monitoring of
warfarin therapy is challenging due to the fact that warfarin exhibits numerous interactions
with other drugs and a variety of factors that influence the dosing of warfarin.
Objective: The general objective of this study was to investigate the prevalence of drugs
prescribed with warfarin that may have a potential drug-drug interaction (DDI) with warfarin.
Methods: This was a cross-sectional, observational or qualitative study that was conducted
on medicine claims data of a pharmaceutical benefit management company for patients
receiving warfarin therapy for a six year period, ranging from 1 January 2005 to 31
December 2010. Drug products that were co-prescribed with warfarin were also identified
from the medicine claims database. The total number of prescriptions for all drug products
during the study period were analysed and compared to the warfarin dataset. This was done
by means of the SAS 9.1® computer package (SAS Institute, 2004). The total number of
prescriptions and medicine items claimed from the database during the study period were
respectively 49 523 818 and 118 305 941. Potential DDls between warfarin and coprescribed
drugs were identified and classified according to a clinically significant rating. The
clinically significance ratings of potential DDls are described in three degrees of severity,
identified as major, moderate and minor (Tatro, 2011 :xiv).
Results: The database consisted of 427 238 warfarin prescriptions and 427 744 warfarin
medicine items, which represented 0.9% of the total number of prescriptions and 0.4% of
total number of medicine items. The total number of patients who claimed warfarin
prescriptions through the database represented 0.9% (n=68 575) of the total number of
patients who claimed prescriptions in the total database (2005-2010). General practitioners
prescribed the highest frequency of warfarin medicine items, representing 58.3%
(n=249 202) of the total number prescribed. The age group that claimed the highest frequency of warfarin prescriptions (n=327 592, 76.6%) and the highest frequency of
warfarin medicine items (n=327 984, 76.7%) was age group 4 (consisting of patients 59
years and older). The distribution between females and males regarding warfarin
prescriptions claimed (n=205 999, 48.2%; n=221 117, 51.8%) and warfarin medicine items
claimed (n=206 232, 48.2%; n=221 390, 51.8%) were almost equal. General practitioners
prescribed the highest average PDD (7.01 mg ± 9.86 mg) of warfarin medicine items.
Paediatric cardiologists prescribed the lowest average PDD (4.61 mg ± 1.29 mg) of warfarin
medicine items. A d-value of 0.1 indicates that there is no practical difference of the average
PDD between general practitioners and paediatric cardiologists. The average PDD of
warfarin medicine items between females (6.60 mg ± 9.06 mg) and males (6.74 mg± 8.41
mg) was almost equal. The age group who was prescribed the highest average PDD was
age group 2 (consisting of patients 20 years to 39 years old) (7.42 mg± 7.42 mg). Age group
4 (consisting of patients 59 years and older) (6.50 mg± 8.90 mg) was prescribed the lowest
average PDD of warfarin medicine items. A d-value of 0.1 indicates that there is no practical
difference of the average PDDs of warfarin medicine items between these two age groups.
The results revealed that drugs with a significance rating (SR) of 1 (n=155 066, 43.3%), 2
(n=30128, 8.4%), 4 (n=137144, 38.3%), and 5 (n=36144, 10.1%) were co-prescribed with
warfarin in the six year study period. The five drugs that was co-prescribed with warfarin
most frequently was aspirin (n=48 903, 13.6%), thyroxine (n=33 954, 9.5%), amiodarone
(n=25 056, 7.0%), simvastatin (n=19 070, 5.3%) and celecoxib (n=10 794, 3.0%). These five
drugs have a SR of 1.
Conclusions: This study showed that the top five drugs most frequently prescribed with
warfarin are aspirin, thyroxine, amiodarone, simvastatin and celecoxib. These drugs can
potentially interact with warfarin. The potential interactions of these drugs are rated with a
significance rating of 1. This concludes that drugs that can potentially cause life threatening
effects and permanent damage are commonly co-prescribed with warfarin. Clinical data
concerning the INR or PT must be obtained in order to evaluate whether or not warfarin
therapy is changed when a potentially interacting drug is co-prescribed. The age of the
patients as well as the duration of warfarin treatment should also be obtained in order to
assess whether warfarin treatment is changed with the progression of age. / MPharm (Pharmacy Practice), North-West University, Potchefstroom Campus, 2013
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/11751 |
| Date | January 2012 |
| Creators | Blaauw, Stephanie |
| Source Sets | North-West University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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