The drug interaction between mercaptopurine and warfarin is documented, but case reports of the existence or magnitude of this interaction are rare. An increased warfarin dosage was required for a patient receiving 12-week cycles of mercaptopurine for acute promyelocytic leukemia. At the start of mercaptopurine therapy, an upward titration of 25% beyond the warfarin maintenance dosage was required to achieve a therapeutic international normalized ratio. When a cycle of mercaptopurine was completed, a sharp reduction in warfarin dosage was required. The mechanism behind this interaction is unclear. Mercaptopurine may inhibit gastrointestinal absorption of warfarin, or it may induce hepatic enzymes that metabolize the anticoagulant. With dramatic changes in warfarin or anticoagulant requirements, practitioners should be aware of potential thromboembolic sequelae or bleeding complications when these agents are prescribed concomitantly. Frequent monitoring and careful dosage titration are warranted during concomitant administration.
Frequency jumps are common in rubidium frequency sources. They affect the estimation of user position in navigational satellite systems. These jumps must be detected and corrected immediately as they have direct impact on the navigation system integrity. A novel weak frequency jump detector is proposed based on a Kalman filter with a multi-interval approach. This detector can be applied for both "sudden" and "slow" frequency transitions. In this detection method, noises of clock data are reduced by Kalman filtering, for accurate estimation of jump size with less latency. Analysis on in-orbit rubidium atomic frequency standard (RAFS) phase telemetry data shows that the detector can be used for fast detection and correction of weak frequency jumps. Furthermore, performance comparison of different existing frequency jump detection techniques with the proposed detector is discussed. A multialgorithm-based strategy is proposed depending on the jump size and latency for onboard navigation satellites having RAFS as the primary frequency source.
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