The incidence of acute kidney injury (AKI) associated with ultramarathon running is varied partially because onsite measurement of serum creatinine concentration (Cr) is rarely performed at races due to logistical challenges. We conducted a prospective observational study using blood samples from volunteer athletes before and immediately after the Leadville Trail 100 Run in August 2014. The goals of our study were to determine the incidence of those meeting the AKI criteria immediately following the ultramarathon race and to evaluate the utility of using medical equations to back‐calculate creatinine levels for runners without known baseline creatinine levels and to identify possible risk factors for developing AKI. We were able to collect pre‐race and post‐race blood samples on 64 and 84 volunteers, respectively. AKI was defined as a Cr increase (≥1.5 times or ≥0.3 mg/dl from baseline). Of the 37 runners for whom we had both pre‐ and post‐race blood samples, 18 (49%) met the criteria for AKI. Using two different methods of back‐calculating baseline estimated creatinine (a fixed eGFR [estimated glomerular filtration rate] and an aged‐based eGFR), 6 (16%) and 22 (59%) of 37 runners met the criteria for AKI, respectively. We found that there is no accurate way to estimate baseline Cr in this population.

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Updated information was obtained on current number of biological children, marital status, and average weekly running distance in the prior 12 months, and the Motivations of Marathoners Scales was completed. Results.-Among the 1349 runners receiving the question, 349 (25.9%) answered "yes." Those answering "yes" compared with those answering "no" were older (median age, 47.3 vs 43.3 years, P o .0001), more likely to be married (76.2% vs 69.6%, P ¼ .019), had more children (median number 2 vs 1, P ¼ .0095), and ran less (median 48 vs 56 km/wk in past year, P o .0001), but did not differ in sex (70.5% vs 67.6% men, P ¼ .35). The Motivations of Marathoners Scales showed significant group differences with those answering "yes" to the question having a higher health orientation (median 5.7 vs 5.3, P o .0001), and lower personal goal achievement (median 5.0 vs 5.2, P ¼ .0061), psychological coping (median 4.4 vs 4.8, P o .0001) and life meaning (median 4.6 vs 4.9, P ¼ .0002) scores. Conclusion.-Ultramarathon runners find benefit from participating in ultramarathon running to the extent that most indicate they would not stop doing it even if it was bad for their health. Not surprisingly, those indicating they would stop were older, more likely to be married, had more children, were running less, were more health oriented, were less achievement oriented, and had less psychological motivations for running.