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
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A lot of blood tests needs the sample to be prepared in a specific way. All blood needs preparation in some way to avoid it coagulating in the test tube. But this might damage some of the things they are looking for. If you pay attention the next time you get your blood drawn you may notice that the test tubes they use are not completely empty but have a tiny droplet of something in them. This is what gets mixed with the blood as it is drawn. And different tests need different things to prepare the blood sample.
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Because we usually don’t need all. And getting them all would just be a burden on the individual. But there are times when we do them collectively in one blood sample. Such as post-partum (delivery) bleeding. We draw a single large enough sample for 5 different tests.
A blood test looks at the number of molecules or ions in a specified volume of blood.
To get this information, you need to mix the blood with different chemicals to draw them out before processing them. This is why different blood bottles exist, because mixing with the wrong ‘reagent’ will produce errors.
Even the most basic (common, not necessarily easiest) blood test like kidney function is still looking for four things: urea, creatinine, sodium and potassium. They all get processed off the same bottle helpfully, but if you wanted to do a full blood count (haemoglobin, white cells etc etc) that’s a different bottle and will give you upwards of 8 different numbers depending on the lab.
TL:DR – most blood tests are actually lots of results rolled into a list. There a far too many things in the blood to look for everything every time, it would cost tons.
Basically, each blood vial has a colored top telling the techs what the blood is going to be tested for. The top tells them what chemicals are in it and therefore what it is going to do to the blood to get the data they need (stuff like coagulents, anticoagulents, stuff that would conflict witheach other if combined in one test)
I work in a lab in a hospital. The short answer is it’s expensive to test for everything. And often not necessary either. The longer version is multifaceted and complex.
First off, blood is full of so many different things and the way to measure all those things is different too. Using an Ion Slective Electrode with different cells can tell you the electrolyte concentrations (although it’s three different galvanic cells – one for Sodium, one for Potassium, one for Chloride). And there’s different reagents for protein, albumin, enzymes like ALT or ALP, CK, troponin (a marker used to detect heart damage), and lots of others. When I do maintainance on the chemistry analyzer there’s dozens of little bottles of reagents to manage. Because take the same sample, mix it with Bromcresol Green, shine a light through it and you can measure the amount of albumin in the sample. But it will only tell you that. You have to take another aliquot (science word for small sample) and mix it with other chemicals to tell you other things.
And there are different color blood tubes to tell you other things. Lavandet tubes have EDTA in them and are used in hematology but aren’t really good for chemistry. EDTA pulls calcium out of the blood to arrest the clotting process so we don’t really use EDTA tubes in chemistry since measurement of calcium is important. In hematology it’s about measuring the amounts of the components of blood. Red cell count, white cell count, platelet count, hemoglobin content, the relative proportion of each type of blood cells (referred to as a differential).
Our formulary in the lab (kind of a guide to different tests) has so many hundreds of different kinds of tests that it would be a huge waste of resources to order everything on everyone. Someone comes in with chest pain to the ED. They will likely order a troponin level, along with some sort of metboloic panel (either a BMP, CMP, or a rental function panel which is really a BMP with a couple additional tests), along with a CBC, maybe a lactate too. Potentially a PT/INR or aPTT too. Basically the doctor looks at the symptoms and uses the blood tests as kind of a guide as to where to go next. Troponin came back negative? Probably not a heart attack. You wouldn’t typically order a Hebatits B Surface Antigen test if the person had no reason (or symptoms) that match the diagnostic picture. If we wanted to run every test in the formulary on a patient we might bleed them dry drawing 100 different tubes. Not literally but it’s like what’s the point?
And as doctors get more information, say the symptoms evolve, or a test points in a specific direction, they add on tests all the time after the fact. But like if a person has no symptoms of diabetes, why bother ordering a Hemoglobin A1C if you know they aren’t diabetic? Or at least aren’t at any risk of having it.
Edit: also I don’t think there’s a lab out there that tests for everything. I work in a ‘full service lab’ in a community hospital that is part of a large hospital system in my area. Something like dozens of facilities, some 30,000 employees across the system. We don’t do everything though. We don’t have a microbiology laboratory in our lab so all our micro samples go downtown. Other more specialized tests also go downtown to the core lab. Ferritans, Vitamin B12s, procalcitonins, etc. But not everything goes to the core lab either. We have a variety of sendouts that go to other reference labs in the area. I think it’s just not feasible to have enough equipment to test for everything everywhere.
One reason is batch testing.
The testers take 10 different vials of blood from 10 different people, pour them all in together and test that. It’s a lot more efficient that testing each vial independently.
Most of the time, the mixed batch comes out fully negative for whatever they’re testing (HIV for example). If the whole batch is HIV-negative, then all 10 individuals are HIV-negative, and we only had to run 1 test instead of 10, a 90% time savings.
But if the whole batch tests positive, then you’ll need a second vial of all those blood samples to test individually. And even then, you only had to run 1 extra test overall.
Edit: Apparently this is more of a Blood Bank process (which is where I heard about it), and less of a “figuring out if the patient has lupus” process.
1) as you said, there are different “markers” we’re looking for. These markers may be specific for a certain organ or disease or may be non-specific (such as acute phase reactants that increase in ANY inflammatory state)
2) the things we measure in our body have different testing principles. For example: we can measure proteins by serum electrophoresis to detect diseases such as multiple myeloma while we would have to use isotope dilution mass spectrometry to measure Urea.
3) another important thing to consider is: there are many tests whose functions may overlap. So we can’t really have a one size fits all type of test since some diseases may have similar results and it would be extremely difficult to diagnose the patient with just one vague test.
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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