Background: Haematohistopathologists require a sample of sufficient volume and quality to be both sensitive to recognising the presence of pathology and to make a specific diagnosis. An 8G trephine needle has a 3.43 mm internal diameter, larger than the 11G at 2.39 mm, which translantes to a 2x larger area. Not only can this provide substantially more material, it may allow for a more robust sample, less likely to suffer a crush artefact. However, the 8G needle can be intimidating, and there are concerns for patient tolerance and bleeding risk. There is no study comparing trephine gauge, so we performed a prospective observational study comparing these two sizes. Aims: A prospective study comparing use of 11G and 8G trephine needles on resultant sample quantity and quality, crush artefact, procedure ease, ease of sample extraction, haemostasis and diagnostic yield. Methods: Practitioners performing bone marrow biopsies at a large teaching hospital selected either 11G or 8G trephine needles for the procedure. They completed a questionnaire on the ease of performing the biopsy, whether several attempts were taken, if there was any prolonged bleeding, the ease of sample extraction and the approximate length. A haemato-histopathologist completed a questionnaire commenting on whether the sample was crushed, diagnostic, sufficient and the overall quality. These results were compared using the R numerical language. Results: Data was collected on 14 11G and 20 8G biopsy attempts, all using a 4" needle. The length of samples obtained were comparable (mean 2.04 vs 2.07 cm, 11G vs 8G). Ease of procedure appeared to be higher in 11G (86%) vs 8G (75%). However, ease of sample extraction was lower in 11G (79%) vs 8G (95%). A further attempt was required in 21% of 11G and 15% of 8G. Regular haemostasis was achieved less in 11G (86%) than in 8G (95%). Haemato-histopathologist assessment revealed slightly less diagnostic samples from the 11G (85%) than from 8G needles (90%), not including failed attempts. Crush artefact was seen in 23% of 11G vs 25% of 8G. Overall quality was comparable between 11G (mean 3.9 of max score of 5) and 8G (mean 4.0). Sufficient tissue quality was lower in 11G (79%) vs 8G (90%). Haemorrhage artefact was lower in 11G (8%) vs 8G (20%). Summary/Conclusion: Our continued study comparing 11G and 8G trephine needle suggests a similar overall sample quality between the needle sizes. With around double the area of sample, theoretically only half the length is needed to obtain the same volume of tissue. Despite this, operators were obtaining similar lengths with both biopsies. Some differences were seen, with the 11G needle holding advantages in the ease of taking the sample, and having less haemorrhage artefact. The 8G needle was easier to extract the sample from, had less repeated attempts, was diagnostic in more cases and more often provided sufficient tissue quantity. Haemostasis appeared to be slightly higher in the 8G biopsies, though this may be due to operator selection based on patient characteristics....