The relationship between previous mild traumatic brain injury/injuries (MTBI) and recovery from a subsequent MTBI may be complex. The present study investigated three factors hypothesized to influence this relation: (1) the number of prior MTBIs, (2) the interval between MTBIs, and (3) the certainty level of previous MTBIs. The study design was retrospective cross-sectional. Participants (N=105) were evaluated at a concussion clinic on average 1 month after sustaining an MTBI, defined by World Health Organization diagnostic criteria. Approximately half the sample had at least one previous MTBI. Subgroups with 0, 1, or 2+ previous MTBIs did not differ in levels of current post-concussion symptom reporting on the British Columbia Post-Concussion Symptom Inventory. Time since the most recent previous MTBI was significantly associated with current post-concussion symptom reporting. This relation was best characterized as logarithmic; i.e., the impact of previous MTBI(s) lessens exponentially as time elapses to a subsequent MTBI. Defining previous MTBIs with a higher certainty level (i.e., probable versus possible) was not consistently associated with greater post-concussion symptom reporting. In conclusion, participants with multiple MTBIs did not report more post-concussion symptoms than those with no history of MTBI. Previous MTBI(s), however, were associated with increased symptom reporting from a subsequent MTBI to the extent they occurred closer in time. Having one or two previous remote MTBIs was not associated with worse outcome from subsequent MTBI in this sample.

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