FAQs: Important Principles for Air-Drying Molecular Assays

Air-drying is an evaporation process where water is gradually removed from the material. It requires a precision oven and optimization of both the temperature and time used to achieve sufficient drying. Each mix has a specific ideal moisture loss level that, once achieved, indicates drying is complete. Guidelines for the oven temperature and drying times are provided in the product handling guides. Optimization will be required as many factors can impact the evaporation process such as the vessel used, the volume per reaction well/tube, the total volume per lot and the final formulation of the mix (with primers and probes).

Q: Can I use the Lyo-Ready mix for air-drying?

No, air-drying requires specific stabilizers, excipients and preservatives which are different from those required for lyophilization.

Q: Is it possible to adjust the oven temperature and do high temperatures damage the polymerase or reverse-transcriptase?

Yes, we recommend optimizing the oven temperature and drying time for each new assay formulation (mix including primers and probes), vessel used and lot size. High drying temperatures may affect the integrity of the mix and its components, including enzymes.

Q: How do you know once drying is complete?

Ideally, 70% moisture should be removed from Air-Dryable 1-step RT-qPCR Mix (MDX095) and 95% from the Air-Dryable Mix (MDX082). However, the optimal moisture loss requires optimization for each assay. Retaining too much moisture may impact the shelf-life of the assay and over-drying may make the mixture difficult to rehydrate and cause a loss of performance. Moisture loss can be calculated by comparing the weight of the initial wet mix in the vessel to the dry mix (refer to the basic workflow for further information). Please note that the air-dried material must be packaged immediately after the drying cycle.

Q: Is it easy to rehydrate an air-dried mix? Do you need to vortex?

The air-dried mix rehydrates in seconds after the addition of the sample. We do recommend, if possible, to gently shake/mix the vessels in order to resuspend the reaction mix before running the reaction, or alternatively mix the solution when adding the patient sample.

Q: Why do I sometimes see a lower Ct value when I compare the performance of the re-hydrated dry mix against the wet mix?

If a mix becomes over-dried, the integrity of the enzymes will be compromised. Conceptually, the ideal “dryness” of a mix will be the highest percentage of moisture loss achieved without losing assay performance. If the Ct value of your dry mix is lower to the comparable wet mix, then the mix has been over-dried and further optimization is required.

Q: Do different probes affect the performance of the assay after the mix is air-dried?

We did not observe any effect of the fluorophore type (Cy5, FAM, JOE or ROX) on the performance of the mix after air-drying.

Q: Will the air circulation from the fan in the oven cause contamination?

We did not observe any contamination during air-drying however we do recommend good laboratory cleaning practice to minimize possible environmental contamination. Examples include surface cleaning or allocating separate locations for the oven drying, qPCR reaction setup, and analysis. In addition, we recommend maintaining the oven following the cleaning instructions provided by the manufacturer.

Q: Could the fan air bring traces of primers/probes from one tube during the drying to another tube?

We did not observe any cross contamination from fan-forced oven drying. Given the mix resides at the bottom of the vessel, it is very unlikely that cross-contamination can occur between the wells. In addition, in accordance with good manufacturing practices, we would discourage oven drying two different types of assay in the same drying cycle.

Q: Do I have to use 8-well PCR tube strips?

Other vessels can be used such as plates, vials or various microchips, but the drying conditions for each vessel/assay would need to be optimized. Once the ideal conditions and parameters are established, they would need to be verified during validation and scale up.

Q: Do you have a list of ovens that you recommend?

We recommend using a precision drying oven (temperature uniformity of +/- 1.5°C) with convection, fan-forced or vacuum capabilities. In our testing, we used a Memmert UF260Plus oven with fan and air flap settings at 100% and 20% respectively. Please note that different ovens will have different specifications and adjustable parameters that will need to be optimized based your assay, reaction volume, vessel type, batch size, relative humidity, and altitude.

Q: Can I simply put the tubes back in the oven for longer incubation if the moisture loss is not enough, or add water if it is too much?

If the first attempt is not successful and does not provide the correct moisture loss value, we do not recommend putting the tubes back in the oven for additional drying or adding water. We recommend re-starting the experiment in order to control all of the variables so that they are reproducible. In addition, opening the door during the drying process is also not recommended unless it is done in a controllable manner and will be part of the process moving forward.

Q: Would fluctuations in ambient humidity and temperature where the drying oven is located affect the drying protocol, such that the moisture lost is outside the 70% range?

Unlike the effects with lyophilization, there is no impact of the environment to the moisture loss with air-drying.

Q: Can I test using different vessel types and/or different reaction volumes per well in the same drying cycle?

We recommend that different drying cycles are performed to test the effect of altering any assay variable such as vessel type, reaction volume, primer/probes, and lot size. This will ensure that the conditions and outcomes are reproducible.

Q: Does the balance need to be next to the oven for quick weighing?

No, the balance does not have to be next to the oven, just close the vessel once it is out of the oven and go to the balance (even if in a different room) for measurement.