Jake Allen made 20 saves for the Blues, who fell out of playoff position during a seven-game losing streak from Feb. 11-27. Allen has stopped 92 of 97 shots in the three games since taking over for Carter Hutton, who has a sore neck.
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"We've still got a lot of work to do," Blues coach Mike Yeo said. "We're still on the outside looking in, but a little bit closer."
Corey Perry and Derek Grant scored for the Ducks (34-24-12), who have lost three straight and are tied with the Avalanche for the second wild card with 80 points but have played two more games. The Los Angeles Kings moved one point ahead of Anaheim for third place in the Pacific Division with a 3-0 win against the Vancouver Canucks on Monday.
John Gibson made 19 saves for the Ducks, whose nine-game point streak at home ended (7-0-2).
"It's not a frustration with our effort," Ducks captain Ryan Getzlaf said. "It's a frustration with the fact that we're having trouble understanding what it takes shift in and shift out every period at this time of year."
Robert Bortuzzo scored just inside the left post to give St. Louis a 1-0 lead at 17:30 of the first period. Brayden Schenn took possession of the puck at the red line, pulled up above the left circle and sent a pass to Pietrangelo, who had just crossed the blue line. Two defenders went to Pietrangelo and he swept a backhand pass to Bortuzzo coming down the middle alone.