Introduction: Laboratory turnaround times (TAT) influence length of stay for emergency department (ED) patients. We studied biochemistry TATs around the implementation of a plasma separating tube (PST) that omitted a 20-minute clotting step in processing when compared to the standard serum separating tubes (SST).Methods: We compared laboratory TATs using PST vs SST in a prospective before-and-after study with a washout period. TATs for creatinine, urea, electrolytes, troponin, and N-terminal pro b-type natriuretic peptide (NT-proBNP), as well as hemolysis rates, were collected for all ED patients. Results were excluded if the TAT was four minutes or less (data entry error). We recorded the 90th percentile response times (TAT90; the time for 90% of the tests to be completed). Statistical analysis used survival analyses, Mann-Whitney U tests, and Chi-square tests of independence.Results: SST and PST groups were matched for days of the week, critical values, or hemolysis. There was a statistically significant reduction in median TAT and proportion completed by 60 minutes. However, the effect size was only two to four minutes in the In-Lab-TAT90 with the PST tubes for all tests, except B-type natriuretic peptide (BNP).Conclusions: Reducing the machine processing time for stat blood work with PST tubes did not produce a clinically meaningful reduction of TAT. Clinically important improvement for Lab TAT requires process analysis and intervention that is inclusive of the entire system. Fractile response times at a 90th percentile for TAT within 60 minutes may be an accurate benchmark for analysis.