SHOULD PATIENTS WITH
A HISTORY OF SHINGLES
RECEIVE ZOSTAVAX?
The varicella zoster virus (VZV) causes two distinct clinical conditions. Primary infection with VZV results in varicella (chickenpox), while reactivation of latent VZV in the sensory ganglia causes herpes zoster (shingles). Two separate vaccinations are available for the prevention of these diseases. The varicella vaccine (Varivax) is used to prevent chickenpox, and the zoster vaccine (Zostavax) is used to reduce the risk of shingles and postherpetic neuralgia. Although both vaccines contain the same strain of live, attenuated varicella zoster virus, they are not interchangeable and contain different doses. Additionally, they are both contraindicated in patients who are pregnant or immunocompromised.
The Advisory Committee for Immunization Practices (ACIP) recommends administering one dose of the zoster vaccine (Zostavax) in patients 60 years and older, regardless of whether there is a history of herpes zoster (shingles). The clinician does not need to inquire about a previous history of chickenpox or obtain serologic evidence of varicella immunity prior to vaccine administration because approximately 99.5% of the US population older than 40 years has serologic evidence of previous VZV infection.1
The varicella vaccine (Varivax) is not indicated in most adults because people born in the United States prior to 1980 are considered immune (two exceptions include pregnant women and health care workers). However, people born elsewhere are not routinely considered immune, and an adequate history should be obtained with serologic testing if indicated. Any person without evidence of varicella immunity should receive two doses of the varicella vaccine, regardless of age. The zoster vaccine is not recommended for people who have previously received the varicella vaccine. Health care providers are unlikely to encounter patients who are eligible for the zoster vaccine (older than 60 years) who have also received the varicella vaccine. Asking about history of varicella immunization in patients eligible for Zostavax is therefore unnecessary at this time.1
WHEN SHOULD DABIGATRAN BE STOPPED PRIOR TO
INVASIVE PROCEDURES?
Dabigatran (Pradaxa) is a direct thrombin inhibitor that was recently approved for stroke prevention in patients with atrial fibrillation. The approval of a new oral anticoagulant was exciting; however, many practical questions are now arising regarding its use.
According to the Pradaxa package insert, dabigatran should be discontinued 1 to 2 days prior to invasive procedures in patients with a creatinine clearance of 50 mL/min or higher. For patients with a creatinine clearance less than 50 mL/min, the drug should be held for 3 to 5 days. Longer times should be considered for patients undergoing major surgery, spinal puncture, or insertion of a spinal or epidural catheter. To understand approximately when dabigatran should be stopped prior to procedures, review of basic pharmacokinetic principles such as half-life is also helpful.
The half-life is the time it takes for 50% of a drug to be eliminated from the body. Practically speaking, a drug is considered "completely" eliminated after 5 half-lives (Table 1). However, many clinicians consider a significant removal of drug to have occurred after 4 half-lives. Conversely, a drug may still have pharmacodynamic effects on the body even after it is eliminated (consider the antiplatelet effects of aspirin, which last the life of the platelet).
Dabigatran has a half-life ranging from 12 to 17 hours in healthy subjects.2 This means that "complete" elimination of the drug could take anywhere from 60 to 85 hours. However, 75% of dabigatran may be removed as early as 24 hours after the last dose (assuming a half-life of 12 hours). In patients with chronic kidney disease stage 4, the half-life may be as long as 27 hours.
Unfortunately, no convenient method is available to monitor the anticoagulant
effects of dabigatran. Dabigatran prolongs the activated partial thromboplastin time (aPTT), ecarin clotting time (ECT), and thrombin time (TT).2 The international normalized ratio (INR) is unreliable and may or may not be elevated. If determining whether a patient is still anticoagulated is critical, the ECT is probably the most reliable. However, an elevated aPTT or TT is also indicative of remaining anticoagulant effect. JAAPA
Larissa DeDea, PharmD, BCPS, PA-C, is a clinical pharmacist with Northern Arizona Healthcare, Flagstaff, Arizona. In addition to being board certified in pharmacotherapy, she is a graduate of the Yale University PA Program.
REFERENCES
1. Harpaz R, Ortega-Sanchez IR, Seward JF. Prevention of herpes zoster: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep. 2008;57(RR-5):1-30.
2. Pradaxa [package insert]. Ridgefield, CT: Boehringer-
Ingelheim; 2011.