Atmospheric pressure changes and unexplained variability in INR measurements.

Changes in atmospheric pressure may influence hepatic blood flow and drug metabolism. Anecdotal experience suggests international normalized ratio (INR) variability may be temporally related to significant atmospheric pressure changes. We investigated this potential association in a large sample of patients with multiple INRs. This is a retrospective review of outpatient anticoagulation records from the Iowa City Veteran's Affairs Medical Center and affiliated outpatient clinics from October 1999 to July 2007. All patients, receiving at least one prescription for warfarin and INR at least 30 days or more from the date of the first warfarin prescription, were identified. INRs during periods of hospitalization and vitamin K use were excluded. Proximity analysis using geocoding of ZIP codes of identified patients to the nearest National Oceanic and Atmospheric Administration station was performed to assign atmospheric pressure with INR. Spearman's Rho and Pearson's correlation were used to evaluate atmospheric pressure and INR. Unique patients (1441) with 45 187 INRs were analyzed. When limited to nontherapeutic INRs following a previously therapeutic INR (1121 unique patients/5256 INRs), a small but clinically insignificant association between delta INR and delta atmospheric pressure was observed (r = -0.025; P = 0.038), but not for actual INR and atmospheric pressure (P = 0.06). Delta atmospheric pressure demonstrated greater variation during fall/winter months compared with spring/summer (0.23 vs. 0.15 inHg; P < 0.001); however, variability in INRs for the corresponding seasons was not significant (P = 0.136). No significant difference was detected in the proportions of nontherapeutic INRs among the different seasons (P = 0.371). No correlation was observed between atmospheric pressure changes and INR variability. These findings refute the anecdotal experience seen in our anticoagulation clinic.