(1110-C) Assessing the Efficacy of Pressure-Based Monitoring in Automated Liquid Handling

Abstract: Blood-based diagnostic tests typically require sample preparation steps to remove interferences and select for biomarkers of interest. As such, an initial pre-analytical step involves accurately transferring a set amount of each specimen into a secondary tube for processing. Robotic liquid handlers are often used to automate this procedure (along with subsequent reagent additions) to provide efficiency and scalability despite suffering from inconsistencies due to clotting and/or frothing – particularly for plasma specimens. To that end, we empirically characterized the performance of Total Aspiration and Dispense Monitoring (TADM™) technology on a Hamilton® liquid handler to determine the robustness of pressure-based monitoring for the accurate transfer of human plasma routinely received in a clinical laboratory.

Remnant, deidentified EDTA plasma specimens were stored frozen (< -10 °C) and thawed at room temperature on the day of use. Immediately before pipetting, specimens were mixed on a multi-tube vortexer and loaded into sample carriers. A Hamilton® Microlab® STAR™ equipped with twelve 1000 μL CO-RE® I pipetting channels was setup with 300 μL CO-RE® II conductive filter tips and an on-deck analytical balance to perform gravimetric measurements. The system was programmed to transfer 50 μL from each specimen tube onto the balance and record the weight along with the TADM™ curves for both aspirate and dispense steps.

Over three days, we processed 576 unique plasma specimens and measured on average 45.4 μL (density = 1.023 g/mL) with 26% CV. Many specimens were under-recovered with 46 transfers (8%) measuring less than 25 μL (-50% bias). Allowing for ±10% bias in the gravimetric measurement relative to the expected volume (50 μL), we categorized specimens transfers as true positives (TP; < ±10%) or true negatives (TN; ≥ ±10%). Using this approach, 91 transfers (16%) were TN-classified and had an average of 22.4 μL (76% CV), while 485 transfers (84%) were TP-classified with an average of 49.7 μL (2% CV).

An R script was used to extract TADM™ curves and relate the data with gravimetrically-determined classifications. Reference curves for aspirate and dispense steps were defined by the average pressure value at each timepoint from the TP-classified transfers. Various pressure tolerances surrounding reference curves were modeled and presented herein. For example, a static window (±500 Pa) around each reference timepoint resulted in 500 specimen transfers (87%) with an average gravimetric result of 49.1 μL (6% CV). Applying this static threshold gave 99% sensitivity and 90% specificity compared to gravimetry – highlighting the capability of TADM™ to correctly identify inaccurate pipetting events if tolerances are properly configured. Taken together, this analysis provides important guidance to researchers when developing liquid classes given the heterogeneity of plasma specimens commonly experienced.