Previous Creatinine Levels Safely Predict Amantadine Dose for Influenza a Outbreak Control

Abstract

BACKGROUND: Amantadine, an antiviral agent, is the only drug currently approved in Canada for prophylaxis of influenza A virus infection. To minimize side effects, the amantadine dose is adjusted for age and estimated creatinine clearance (CrCl) based on plasma creatinine (Cr) levels. As amantadine is used more frequently for influenza A outbreak control in care facilities for elderly people, physicians are increasingly called on to prescribe it for residents and to consider the necessity of requesting plasma Cr levels. OBJECTIVE: To determine whether previous Cr levels are predictive in estimating current CrCl and safe amantadine dose determination. DESIGN AND SETTING: Residents' charts were reviewed in two facilities in Vancouver, British Columbia. CrCl estimated using previous or current Cr results, current weight and age, as well as recommended amantadine doses based on Canadian National Advisory Committee on Immunization guidelines, were studied. RESULTS: 165 charts with Cr results in March 1998 were included; 122 had results before March 1998, and 103 had Cr results after March 1998. Pearson's correlation coefficient for CrCl estimated from current and previous Cr values was 0.929 for results less than six months previously, 0.974 for six to 12 months previously and 0.952 for 12 to 18 months previously. The same or a more conservative dose of amantadine was predicted in 92% of cases when using a Cr result taken within the previous year and in 76% of cases when using a Cr result taken 12 to 18 months previously. CONCLUSION: In long term care facilities, Cr levels measured up to 12 months previously can usually safely be used to estimate CrCl. Using previous Cr results permits advance preparation of doctor's orders for amantadine prophylaxis and avoids repeating Cr testing on every resident when an outbreak occurs, reducing related staff time and cost. Pour le résumé, voir page suivante I nfluenza is a major infectious cause of death in the elderly, with institutionalized seniors particularly at high risk. Amantadine hydrochloride is the only drug approved in Canada for prophylaxis of influenza A virus infection. Because amantadine is used more frequently for influenza A outbreak control in long term care facilities (LTCFs), physicians are increasingly called on to prescribe it for residents and to consider the necessity of requesting plasma creatinine (Cr) levels to determine the appropriate amantadine dose.Amantadine is 70% to 90% effective in preventing illness caused by influenza A viruses (1), and reduces the severity and duration of influenza A illness if administered within 48 h of onset. About 5% to 10% of healthy young adults taking amantadine for prophylaxis report difficulty concentrating, insomnia, light-headedness and irritability. These side effects are usually mild and cease shortly after the prophylaxis is stopped but may be more frequent in the older population unless a reduced dosage is used. Serious side effects (eg, marked behavioural changes, delerium, hallucinations, agitation and seizu...