Why can’t a single blood test show you all the information you need instead of having to take multiple tests for different markers?
In: Biology
This was what the girl proposed with Theranos; “one drop of blood” testing for many different things. We don’t have the science or technology to do this.
There are two main considerations here and thats volume required for analysers and how the analysers detect analytes.
Most medical tests are done using spectrophotography which involves firing a beam of light through a sample that’s been mixed with a reagent and measuring the change in wavelength of the light through the sample over time. The reagent used typically binds to whatever analyte your looking at measuring, for example CRP, so you’ll need to use a different analyte and a different sample for every individual test you want to do.
Analysers have different methods of doing this off a single sample, the one I used to work on had a carousel, your sample would sit in the back of the machine on a belt and a little pipette on an automated arm would suck up the tiny amount of sample and deposit in a bit of the carousel. It would do the reaction in there and measure it and clean it out. This process doesn’t really need much sample however the arm that takes those little samples uses fluid measurement to know how far to go down into the tube. What this means is you need a “dead volume” which is the minimum the arm can detect. If your blood bottle has only a tiny amount the arm will hit the bottom of the tube and break. For this reason, if there’s lots of tests being done we need multiple bottles.
If your talking about the different colour bottles that’s a whole different thing. Most biochemistry and serology analytes are done using serum (normally a gold coloured top in the UK), this is when you take blood then spin it down to remove all the red blood cells after they have been allowed to clot. We do this because red blood cells and big and get in the way of that spectrophotography j mentioned earlier and unless we are measuring them directly there’s no need to have them in there. Many of the tests on your form will be done using serum or plasma, which is the same as serum in its spun down but anticoagulant is added first as whatever we are measuring gets gunked up in the clot otherwise.
Then you’ll have tubes like EDTA (purple top in the UK) these contain an anticoagulant to stop clotting and stablise the red blood cells. This is used for blood counts which are vitally important and are measured usually through a different mechanism, the way we used to do it was flow cytometry which is firing a sample through a water pistol with a nozzle so thin only one cell can fit through at a time and firing lasers at it.
There are other less used tubes then but it’s all about the stability of the sample and if you can allow the sample to clot without losing whatever analyte your looking for. Also it depends on the mechanism used to measure it, although spectrophotometry is common for alot there are other methods that require different stabilisers and anticoagulants.
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medical lab professional here
some tests are done on serum, some plasma, some whole blood that has been prevented from clotting.
The different coloured tubes contain different chemicals. Most of them are to prevent clotting. If I were testing you for a CBC (complete blood count), it would include things like the number of white blood cells per litre of blood. We would not be able to count them if the white cells were trapped in a blood clot (like a net). Also, sucking up small bits of clot might clog up my analyzer and cause it to go down.
One easy-to-understand example of having to have the right tube to have the right sample for a particular test is: grey top is potassium oxalate anticoagulant. How could I tell how much potassium is in your blood, if there is potassium (ox) in the tube before your blood even goes in?
Some tests also require very strict standards regarding the ratio of blood to anticoagulant in the tube (chemical to stop clotting). If you were on coumadin (blood thinner) and I was testing your INR to ensure the amount of coumadin you were taking was keeping you in the INR’s therapeutic range, then the tube would need to be at minimum 95% full of blood. Any less than that and I have to reject the sample. Similarly if the collection was difficult, or performed poorly, and the blood cells lyse (break apart), then the contents of the blood cells will change the amount of different substances in your serum or plasma. For example, red blood cells contain a lot of potassium. If the red blood cells were lysed, they’d release their potassium (etc.) into the serum or plasma making it impossible to tell how much potassium is in your serum or plasma. Imagine an icecube made of orange juice that you put in a glass of water. I’m comparing the icecube to the red blood cells – as the ice melts, or the red cells lyse, they release their contents into the surrounding fluid.
Medical lab professionals study long and hard to know how to get physicians accurate results. Most of the time, nurses and physicians do not have the same specialized knowledge as we do. Sometimes they pressure us to run an inadequate sample. However, a wrong result can be very harmful.
Up to 95% of a patient’s chart is lab results. Support your local lab professionals!
I don’t quite understand – I’m assuming that the poster and all the replies explaining “why” are in the US? And it’s maybe a US specific problem where they can bill more money if they draw more times?
I lived in the UK, and I was seriously ill for a couple of days with what turned out (later) to be food poisoning. Part of the documentation provided to me later after I left hospital was a variety of metrics about various blood markers. There were about 25 markers on the sheet, and they only took my blood twice in the time I was in hospital.
So it’s definitely possible to test for multiple blood markers (examples in mine were things like cholesterol, various levels of different types of blood cells, calcium, glucose, sodium, potassium etc.)
There are quite a few reasons why you don’t test for every marker on every patient, and if the question is why there is no “universal test”, it gets even trickier.
***Cost:*** for each marker you want to test for, you need a specific reagent that reacts with the components of your blood in a very specific way. If you tested for literally everything on everyone every time, the sheer amount of reagents used would counter any proposed benefit in simplicity. Testing for syphilis on literally every patient would be extremely wasteful, and even more so if you did it every day on every patient who ever had prolonged hospital stay.
***Overlap in reading areas (wavelengths):*** If what you’re asking for is why there is no “universal analysis” that just reads every substance at the same time, there are many reasons. One of which is that a lot of tests are interpreted based on how the sample interacts with light. You add a reagent, which leads to the formation of a compound which reacts in a special and predictable way with light at a certain wavelength. Some other substances are part of reaction chains that are read at the same wavelength, so if you then had a signal spike, it’d be impossible to know which of those substances were the cause. All you’d be able to say is that the concentration of A and B combined is X, but that would be of no use. That is one of the reasons why analyses have to be separated into specific tests for specific substances, so that there is no interference from the other tests being done simultaneously. It’d be a very unfortunate situation to be in to have to say “your rash is either because of allergies or syphilis. We’re not exactly sure which, because we ran the tests at the same time”.
***Tube additives:*** the tests used for analysis have special additives that, among other things, affect how blood coagulates. Some analyses, like cell counts, require a sample that has not been coagulated at all so you can count and differentiate the cells. Other tests need all cells removed, because the presence of cells can over time contaminate a sample (the concentration of potassium inside of cells is 30 times higher than the concentration outside of them). When you have different tests that simultaneously require the removal of- and the inclusion of cells, it gets hard to have one universal tube, let alone an universal test.
There are a ton of other good reasons, but the response is already longer than I wanted. A comprehensive list or explanation for why would just be too dang long to write or read.
Oh oh I’m a phlebotomist and I’m so ready to go!
There are two reasons!
1) what is being tested? Sure we say it’s a blood test but there’s a lot in blood. There’s the cells, antibodies, serum, clotting factors, just a ton of different things, and each tube in a blood draw has different chemicals to make one of those accessible to be analysed
2) you need quantity! It’s unfortunate but we get more reliable results the more blood we get. So sometimes we need multiple tubes for these tests
Edit: words
There actually was a company that promised to sake a small sample of blood and run it through an automated process that did “all” the tests in one go.
That company was Theranos. It was all bullshit.
Some tests can be done together. Some tests just need a good amount of blood to be accurate, and some of those tests involve doing things to the blood that would spoil the results for other sets of tests.
Different tests require samples in different conditions. Most Blood Chemistry tests are done on serum which is the liquid fraction of clotted blood so no anticoagulants are used in the containers for those tests. Blood Count tests require whole blood so they are collected using an anticoagulant that binds up the calcium in the sample which is required for clotting. Blood Sugar tests are collected using and anticoagulant that not only binds up calcium but inactivates the enzymes which may alter the sugar levels.
For some, it’s because the concentration of compounds you’re looking for is so small that a larger quantity of blood is needed. For example, if you’re looking for a compound that occurs at a concentration of 1 molecule per ml of blood, you need enough blood so that you can reliably find enough molecules to confirm the concentration.
For others, it’s because often the act of running a test on a given sample taints the sample and prevents other tests from being run on it, therefore you might need four or five samples to ensure that each test is run on untainted blood.
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