NEET PG / USMLE – MCQ 30
Warfarin is administered to a 56-year-old man following placement of a prosthetic cardiac valve. The warfarin dosage is adjusted to maintain an INR of 2.5. Subsequently,trimethoprim-sulfamethoxazole therapy is begun for a recurring urinary tract infection. In addition to monitoring prothrombin time, which of the following actions should the physician take to maintain adequate anticoagulation?
(A) Begin therapy with vitamin K
(B) Increase the dosage of warfarin
(C) Make no alterations in the dosage of warfarin
(D) Decrease the dosage of warfarin
(E) Stop the warfarin and change to low-dose aspirin
Correct answer : (D) Decrease the dosage of warfarin
Sulfonamides displace warfarin from its plasma protein binding sites, thus potentiating its action. This increases the risk of bleeding. Hence the dose of warfarin must be reduced.
e
c
B
Ans.c
drugs like trimethoprim and sulfmethoxazole PROLONGS PROTHROMBIN TIME, INCREASES INR AND INCREASES THE EFFECT OF WARFARIN LEADING to increase in bleeding. so monitor PT, INR. if INR> 7 stop warfarin. 4.5 – 7 decrese the dose of warfarin. ans might be D.
b
c
answer-d
C
Sulfamethoxazole increases the effect of Warfarin by
1) Decreasing its metabolism
2) Competeing against warfarin at Plasma protein binding
Use of alternative is recommended. (Or reduce the dose)
So Ans: E
d
reduce the dose of warfarin
answer is D
trimethoprim/sulfamethoxazole-induced hypoprothrombinemia in a patient receiving ongoing warfarin therapy, so the warfarin dose should be reduced
option D
E…as -Vitamin K Antagonists (eg, warfarin): Sulfonamide Derivatives may enhance the anticoagulant effect of Vitamin K Antagonists. Risk D: Consider therapy modification (merks manual)
Cotrimoxazole is known to inhibit cytochrome P450, which is involved in warfarin metabolism,so reduce dose ans D