In-vitro tests

Here at the happy medical device factory we've moved on from sterilization to our next crisis, the in-vitro test. There are still quite a few outstanding sterilization issues, but we like to make things interesting by panicking about something else before the last crisis completely subsides. The in-vitro test's goal is to obtain data that supports the safety and efficacy of the medical device.

Usually when a test is described as in-vitro it involves using blood outside of a body. Keep in mind when I describe it, I'm an engineer, so as usual- I probably don't know what I'm talking about. In our case, we run down to the local slaughterhouse and pick up gallons of cow blood, add an anticoagulant, heparin for us, (you can also use sodium citrate) and bring it back to use for our test.

When we get the blood in we run it through an arterial filter, add dextrose (to preserve- that is the theory anyway), check the temperature, pH and hematocrit. The pH we want between 7.2 and 7.4, but it depends on temperature somewhat, we're currently in disagreement about what we should do if the blood comes in above 7.6 or so, I'm in the throw it out and try again tomorrow camp, but others have argued for adjusting it. Luckily, we haven't had any pH above 7.6 in years. The hematocrit (hct) we generally get 38-42% and we adjust it down to 32% by adding saline, a small vain attempt to keep at least one thing consistent across tests. Another common tactic is to adjust hct to 25%. A previous incarnation of this test had us removing the buffy coat layer of the blood, but that required a lot of time and effort that did not really improve the results.

The next step is the moment of truth for the whole day, splitting the blood into the sample groups, for us, our device, predicate device, and control. I say it is the moment of truth because once you split it into the three containers or whatever you're testing in, you test the blood variable you're looking at in the test. You hope and pray that these initial readings of whatever blood variable you're testing come out reasonably close together, if not you just wasted half a day. It sounds easy, but blood is not consistent and any number of small factors can mess things up, blood settling, not perfectly clean containers, water in the sample containers, etc.

Once we have that down the test can begin in earnest for the next 5 or so hours. The reasonable life of blood is generally 6 hours. After that is clean up, it makes for a long day. Entertainingly enough now, on a previous 72 hour device I worked on the FDA required us to do an in-vitro test for 72 hours, saline tests weren't good enough for them, the results were predictable, a black, stinky soup that stunk up the place for weeks after it was disposed of, but hey, we got our largely irrelevant results!

My main task after an in-vitro test seems to be discouraging people from comparing results across in-vitro tests, but that is a post for another day.