Ensuring that assay data collected in one laboratory is comparable with data collected in another is crucial to the new drug approval process. Liquid handling instruments are the most common cause of assay transfer failure, and errors can come from a variety of sources, such as the liquid handling instruments, operators and environmental conditions in the laboratory. These errors can propagate during assay transfer and significantly affect results.
How to be sure liquid handlers are performing optimally for assays
The role of liquid handling can be critical in effective assay transfer. Keith Albert, product manager, Artel, provides top tips to ensure data assay transfer can be achieved successfully within and across laboratories.
With the life sciences industry racing down the path towards globalisation and outsourcing, the importance of ensuring that assay data collected in one laboratory is comparable with data collected in another is more important than ever. Data collected from all over the world is used in the new drug approval process and development of clinical diagnostics is more frequently an international, multi-laboratory programme.
Ensuring that assays give the same results on different platforms or in different laboratories is one of the most complex activities performed in laboratories. Whether the assay transfer involves scaling up an assay to a higher throughput platform or moving an assay from a development to a quality control environment, assay transfer failures can result in significant delays in projects and manufacturing, additional costs and incomparable data. In today’s laboratory environment where time-to-market pressures, fierce competition and scarcity of resources are commonplace, steps must be taken to prevent unnecessary problems.
To ensure effective transfer, all elements of an assay must be carefully analysed and understood before the transfer is started. While differences in equipment, reagents, technique and interpretation of methodology are typically the focus of assay transfer preparation, less obvious variables, such as liquid handling instrumentation, are often ignored. However, liquid handling instruments are some of the most common laboratory devices and the most common cause of assay transfer failure.
The accuracy and precision of liquid handling instruments, such as pipettes and automated liquid handlers, cannot be taken for granted. Errors in liquid delivery can often go unnoticed, undocumented or misunderstood, and can come from a variety of sources, such as the liquid handling instruments, operators and environmental conditions in the laboratory. These errors can propagate during assay transfer and significantly affect results.
To prevent liquid handling problems and guarantee successful assay transfer, a laboratory can follow three basic steps:
Understand the assay: Variations in the amount of liquid delivered will alter analyte and critical reagent concentrations, which in turn affect the veracity of assay data. To minimise these effects it is recommended that each of the liquid handling steps in the assay is analysed in terms of the parameters listed in Table 1.
|Table 1. Parameters that affect a working assay|
|Plate or tube format (96, 384, etc.)|
|Selected analyte concentration|
|Number and type of dilution steps|
|Number of liquid transfers|
|Number of incubation or centrifugation steps|
|Number of wash steps|
|Liquid properties (viscosity, temperature, etc.)|
|Target or off-set volume|
|Single vs. multi-sequential dispenses|
|Reagent mixing protocol|
|Pertinent liquid class settings|
|Accuracy of volume transfer|
|Overall speed between transfers (time delays)|
|Precision of volume transfer|
|Wet vs. dry dispense|
|Type of liquid handler/pipettor:|
|Types of Tips/Cannulas:|
|Operator skill and technique standardisation|
|Environment, temperature, and humidity|
By precisely describing each liquid handling action, the critical liquid handling steps can be identified and the critical parameters evaluated. This process will highlight any areas of the protocol that could prove problematic during assay transfer.
Implement regular verification and calibration checks: The next step involves evaluation of the accuracy and precision of delivered volumes. Implementation of verification checks prior to assay transfer ensures that the performance of the liquid handling instrument is known and reduces the potential for failure. Additionally, regular calibration at both the transferring and receiving sites further minimises the risk of error. If the performance of the liquid handling instruments at both sites is evaluated using the exact same method, risk of error can be minimised as the results will be directly comparable.
For assay data to be reliable, the amount of liquid dispensed needs to be both accurate and precise. Accuracy refers to how close the amount of liquid transferred is to the target volume, whereas precision tells us how close the repeated measurements in a group are to one another.
If we imagine a shooting target, accuracy relates to how close to the target a shot is, or how close to a target volume the volume dispensed is. Precision relates to how close together a number of shots are, or how much each delivered volume deviates from the others. It therefore follows that a liquid handling instrument could be precisely inaccurate, or imprecisely accurate.
There are a number of methods used to evaluate the volume transfer performance of liquid handlers and pipettes. For example, gravimetric analysis is often used to measure the weight of liquid dispensed using an analytical balance and is adequate for larger volumes such as high microlitre to millilitre quantities. However, the gravimetric method is not as effective at low microlitre and nanolitre quantities and can only determine the average accuracy across multi-well plates.
In contrast, ratiometric photometry provides a simple and effective method for determining both precision and accuracy across a wide range of volumes, and is particularly powerful for volumes as low as 0.01μL. It can also be used to determine the volume dispensed into each individual well of a multi-well plate.
Ratiometric photometry uses a dual-dye, dual-wavelength absorbance method to calculate how much liquid has been dispensed in a single measurement using an innovative application of the Beer-Lambert law. A sample containing both a red and a blue dye is dispensed into each dimensionally characterised well of a microtiter plate. The wells are then filled with a sample of blue dye at the same concentration of that dispensed in the sample. The volume of sample solution dispensed into each well is then calculated and the accuracy and precision of the instrument determined.
This method provides laboratories with an easy-to-use process to verify the performance of any type of liquid handler, allowing device performance to be compared regardless of model, location, or number of dispensing channels.
Develop detailed documentation: Generating effective documentation is another critical part of pre-transfer work. A detailed standard operating procedure (SOP) describing the assay, as it is executed in the transferring site, is key because it provides a standard for training and executing at the receiving site.
Robust and detailed SOPs created during or after assay development at the transferring site provide information that can further define assay steps to reduce problems at the receiving site. For instance, detailing the liquid handling steps in Table 1 will allow the receiving site to focus on details that may have otherwise been overlooked, and will ultimately make a successful transfer more likely.
The Artel MVS simplifies liquid handler performance verification and ensures standardisation among instruments and locations
Another important component is the transfer protocol, which is a formal protocol defining the expectations for the transfer exercise. It provides agreement between the sites on how the transfer will be executed, and defines the criteria for a successful assay transfer. A comparison between the results for identical samples at both sites is required, and an equal number of determinations for each sample at each site are advisable to simplify statistical analysis.
Additional information and documentation from the sender such as regulatory submissions, raw data, material analysis and development reports can be very useful in supporting future regulatory submissions and assay transfers.
In conclusion, liquid handling steps are a controllable but often ignored variable in assay transfers. Understanding the parameters that affect a working assay, implementing regular verification and calibration checks on liquid handling instrumentation, and developing detailed documentation on liquid handling steps contribute to the overall success of an assay transfer and help labs save time and resources, while increasing confidence in results.