The figures, which do no include ongoing reviews, suggest a decrease in new-business activity across almost all disciplines against levels seen in Q1 2017, including advertising (-36.8%), CRM/direct marketing (-57%), media (-40%) and integrated (-47.7%).
Only digital experienced the same number of completed reviews as compared to 12 months previously.
In advertising, only two accounts worth in excess of £20m (Camelot and Bensons for Beds) completed reviews, with large spenders including Asda, Betway and Moneysupermarket.com announcing pitches but not completing them by the end of the quarter.
However, there was very little change in how brands approached pitches, with almost two-thirds (60.7%) opting for "open" reviews, 20.6% awarding their account without a "comparative" review, and 18.7% handing briefs to roster agencies.
Kerry Glazer, chief executive of AAR, said: "While on the face of it, the first-quarter figures look somewhat worrying, it must be remembered that they are being compared with an incredibly busy first quarter of 2017. Equally, as the Pulse report only covers reviews that have been completed, it is too early to come to any overall conclusions as a large number of reviews have yet to make appointments.
"Consequently, it will not be until the end of the first half of the year that we will start to get an indication of the true nature of the new business market, although from our own experience and pipeline, we remain cautiously optimistic."