There has been increasing interest surrounding the ‘biological clock’s' influence on immunological responses. The 2017 Physiology Nobel Prize was awarded to the team that discovered mechanisms regulating circadian rhythms. Currently the effect of diurnal variation on circulating lymphocytes is largely unknown. Previous studies utilised venous blood with varying results (1-3). This study sought to determine whether fingertip blood volumes are viable as opposed to larger venous volumes with the aim that this modified technique would be used to determine the effect of diurnal variation on circulating lymphocytes using flow cytometry. Blood was collected from a female participant and processed with a BD Biosciences IMK Lymphocyte Simultest Kit, containing reagents identifying B-cells, T-cells, helper T-cells, cytotoxic T-cells, and natural killer (NK) cells using either the recommended 100µL of blood and 20µL of reagent, or 25µL of blood and 5µL of reagent. Lymphocyte counts analysed using a FACSVerse flow cytometer were consistent with both methodologies. T-cells accounted for 62.1% of the 100µL sample and 60.9% of the 25µL sample, while B-cells accounted for 8.59% and 10.6% and NK cells were 9.26% and 6.61% respectively. T-cell subset analysis demonstrated identical helper T-cell counts (35.1%) and cytotoxic T-cells of 29.7% and 23.8% for 100µL and 25µL respectively. All values are within the 95% reference range provided by the manufacturer’s manual. This study demonstrates that lower blood volumes are suitable for lymphocytic assessments using flow cytometry providing validation for methodologies using small fingerprick blood collections. When assessing diurnal variation, as multiple blood collections are required, this revised protocol provides a more reasonable request for participants, compared to the more onerous requirement of venepuncture. Preliminary data is being gathered using this method, analysing the effect of diurnal variation on lymphocytes with the data potentially impacting vaccinations, immunological blood studies and autoimmune disease treatment.