Tech Talk – Medical Device Particle Test Method Validation

See the previous Tech Talk for Medical Device Particles.

As part of their focus on particles, the FDA has required test method validation of your particulate testing.  Their method validation is described as follows: 

“You should describe and validate particle counting and sizing methods. We recommend that you introduce a known amount of various particle sizes into the test setup and quantify the amount of particles recovered. The number of particles recovered should closely approximate the number you artificially introduced into the system. For a system to be considered validated, ≥90% recovery should be demonstrated for the ≥10 μm and ≥25 μm size ranges.”

Why do they say this?  In my experience, larger particles settle to the bottom of your sample container.  So if you don’t do a proper validation and sort it all out you will not get an accurate count of the most important particle sizes.  In older tests from various companies I have reviewed this was actually happening.

So where to start with this?  First off, read ASTM F2743 for a general guideline.  You will want to do this testing in house, the test labs (NAMSA, WuXi AppTec, Nelson, etc.) will all do it but it will be expensive, most likely you will have to travel to their site, and you have to validate the model you are using your device with anyway.   So just buy a HIAC 9703, which is the same machine the test labs use, you can call them and confirm- maybe they have upgraded by now. 

You also want a good supply of low particulate water for your testing.  Low particulate water can be just reagent grade water, buy from any chemical supply company for cheap.  You also may have an internal system that can produce low particulate water, just check in your HIAC, although it is probably less effort to just buy the bottles of water.  All testing / flushing and rinsing should be done using this water.

During the testing you will probably want some clean glassware, it is actually not that hard to keep the glassware clean during the testing, rinse a clean container with low particulate water a few times and you’re probably good to go.  Just don’t dry anything with paper towels; you’re better off air or shaking it dry.  The USP standard requires a laminar flow hood, but you’ll probably be okay as long as the room is reasonable clean and you shut down the wood sander before you start.

Then you’ll want to buy some particulate standards so you can do your validation, the only source I’ve found for these is Fisher Scientific, I prefer the Count-Cal particles and will assume you use these particles.  This is where you may want to think about it some or just buy the following sizes 10 um, 15 um, 30 um, 70 um and 100 um.  You can skip the 10 and 100 sizes and still meet the USP and FDA guidance, but if you’re going to validate the method, you might as well do it only once and not worry about it again.  If you read the literature on the particle standards you will see that for example the 15 um size has all particles above 10 um.  So the 15 um size will validate the 10+ um bin, etc.  The 10 um size is nice to have to show that the under 10 bin is working properly (i.e. you are not counting everything).  I’ve seen people assume the 10 um standard is a normal distribution and that the count above 10 um is one half of the total count, I think this is a questionable assumption and it is better to use the 15 um size to validate the 10 um bin.

You will also note by reading the literature that comes with the standards the count is not calibrated, only the particle size.  So you cannot accurately use a particle count value calculated from the standard literature to compare against, you should measure the standard yourself and use that as your baseline.  This is important to understand and you’ll probably have to explain it to many people in management and quality who cannot be bothered to think about it beforehand.

You’ll want to create a custom test using the HIAC software, measure the particle sizes you’re planning on calibrating to (i.e. at least 10, 25, and 50, probably 70 and 100), discard the first run and display total count and run count. 

The literature that comes with the particles standards says to discard the first run on the small particle sizes, which the machine can do automatically.  The literature that comes with the particle standard also says to discard the last run on the large particle sizes, but this assumes you are collecting all liquid in your sample- you cannot do this if you’re mixing with a stir bar.  You can discard the first and last runs manually, or discard just the first run and make sure you leave some sample in the sample container when the runs are completed.

I would set your test up to do 5 runs total of 5 ml each and discard the first and last run you will have to discard the last run manually.  5 runs require 25 ml of sample plus enough extra to clear the stir bar by a bit.  I’m basing my experience off of small devices, if you’re testing another type of device, adjust the volumes as appropriate, but I would not use a smaller volume.

For the test method validation, your tests should go something like this:

        1.   Wash methods, glassware and HIAC

        2.   Water through model / tortuous path (results will be used as baseline)

   3.   Particle standard (start with largest)

        4.   Remaining particle standards

The first test will be low particulate water in the sample container (ideally a 100 ml beaker- see below).  You want to ensure your water is okay, your cleaning method is good, and your HIAC is clear.  The test you just made should be used for all testing here on out, the standard tests don’t include all particle sizes and are of limited utility.  I would perform these steps to test:

        1.  Put beaker on HIAC stand, align with tape so it is consistent every time

        2.  Put clean stir bar in beaker, cover with parafilm, stir at highest speed setting

        3.  Wait for two minutes while stirring (recommended in USP 788)

        4.  Run test

If you get poor results, then you generally want to rinse more, using soap to wash during particulate testing doesn’t always help unless you’re desperate and willing to rinse a lot.  Using IPA can help but also requires a lot of rinsing, IPA should not be used except when needed, do not use it between every test.  Starting with clean glassware and rinsing a lot with the reagent grade water is your best bet.

Let us talk about mixing the samples briefly.  Gently shaking or inverting the samples is not adequate.  Without stirring with the stir bar while the HIAC is testing, larger particles (50+ um) will settle.  You can easily test this using a large particulate standard.  If you test while stirring you will get a higher particle count overall as well, is this from the stirring or did they settle previously and now you’re counting them?  It doesn’t matter as long as your baseline is consistent with your test.

Once you’ve done this, you’ll need to do some thinking.  How are you going to run your particulate tests long term?   Basically this breaks down to what sample volume you can collect, which depends on your model and device size.  I think ideally you want to end up with at least 50 ml of sample.  One half of the sample will be used to flush the model after use, so you have 25 ml of water to work with for the testing.  A 50 ml sample fits nicely in a 100 ml beaker with a stir bar and the HIAC sample collection probe fits in while still allowing stirring.  You’ll want to make sure the probe is not too close to the stir bar as the stir bar does generate particles (or bubbles which are counted as particles).

A typical set up for a catheter would be your appropriate tortuosity model, with fittings on both ends, on the proximal end a touhy borst with a Y fitting, through the touhy borst is a standard guide catheter.  Your device is delivered through the guide catheter, additional accessories devices may be used if used with your product.

Your typical use might end up like this, fill model with 10 ml water, fill guide catheter with 1 ml water, perform simulated use with your device which takes 4 ml of water (obviously varies by device volume).  Flush guide catheter with 10 ml of water, flush model with 25 ml of water.  There is no magic to the quantities, you just want a complete flush of the system you’re testing and to get around 50 ml or more of sample water.  Going forward, I’m just going to assume your water volume is 50 ml.   You may also flush through your device if appropriate, you should be careful extra steps have a tendency to generate particles and if they’re not done clinically, you’re just asking for trouble. 

An example of a simulated use for a catheter would be to prepare your catheter (i.e. remove from packaging and hydrate), place your catheter in the guide catheter, advance and retract over a guide wire through tortuosity several times and maybe deliver a stent.  In this case you would definitely flush through the model as the distal end of the catheter is exposed to circulating blood.  If the clinical use was to flush through the guide catheter, you would flush through the guide catheter as well.  If the clinical use was to flush through the catheter (i.e. a contrast injection), you would flush through the catheter as well.

Before testing your device, verify that your model is not generating significant amounts of particulates and use it as a baseline.  You’ll want to make sure it has no dead zones before starting and it is best if the model can be easily drained.   If you’re using a guide catheter or other accessory device, you may want to include that in the baseline, I would do this.  When choosing what accessory devices to use in your baseline, use clinically relevant devices, but also chose ones that are unlikely to generate particulates.  You want a solid guide catheter and guide wire, not something that generates a large number of particles that can obscure the results from your device.  If the accessory devices generate too many particles you may consider other alternatives, like plain PTFE tubing if that is reasonable. 

Once your model is set up with associated fittings and any accessory devices you want to include in the baseline, inject 50 ml of water through, collect the effluent in the clean sample container and drain the model into the sample container.  Perform your test on the sample, I would repeat three times, average each bin, and use this as your pre-test baseline.  You’ll probably want a post-test baseline as well, or alternatively you could take a pre-test baseline prior to every test.  Your model should really have very few particles and hardly ever one 50+ um.

Some people want to collect all their samples, and then test them all, but I feel pretty strongly that you should test your samples as you obtain them.  Letting them settle in the sample container isn’t going to do you any favors down the road, you’ll probably get low particle counts now, but when you need to do some comparison testing or need to make a change it will be more difficult to reproduce. 

To validate your test and model, take the 70 um particle standard bottle and make a “standard solution”, to make the standard solution:

         1.   Shake the 25 ml bottle of 70 um standard solution vigorously for 10+ seconds

         2.   Pour entire 25 ml of 70 um standard solution into a 400 ml beaker

         3.   Pour 200 ml of LPW into beaker

         4.   Pour 25 ml of LPW into the empty standard solution bottle, rinse and then pour into the beaker

         5.   Put clean stir bar into 400 ml beaker and stir at a moderate speed, do not stop stirring

         6.   Cover with parafilm when not in use

Now you’ll want to test 50 ml of your 70 um standard solution using the procedure above and see what you get at the readout from the 50 um size (the 70 um particulate standard is 100% above 50 um), you should be ballpark of the bottle count at the 50 um size (after you take into consideration the dilution we did) and all of your runs 2-4 should be consistent, use the average of the runs.  You should be about half of the bottle count at the 70 size, but that is less accurate and I wouldn’t sweat it too much.  If you see your last run spike then you’re probably too close to the stir bar and you may want to consider increasing your sample size or raising the HIAC sample intake if possible, if you do this, repeat the test. 

Once you’re happy with those results you can inject 50 ml of the standard solution into your model, collect the effluent and see how you do.  Average the runs from your test (discarding which runs you said you would).  Subtract your baseline result average before you calculate the amount recovered.  You want to recover more than 75% of what you put in per the FDA guideline.  The 70 um particle size is the most challenging, which is why we started here, so don’t worry too much if you don’t get it the first time.  If you’re not recovering at least 75% of your starting particles- your particles most likely have settled in your standard solution.  Turn up your mixing on the standard solution and start over.  You’ll want to proceduralize the mixing.  If you recover more than 110% you probably want to look at your environment and wear a hair net or breathe more through your nose or something.

Repeat with the rest of the particle standards, you want more than 90% recovery with at least the 15 and 30 um sizes and you have finished the particle testing test method validation.

See Part 3: Particulate Testing of Medical Devices.