Why can’t a single blood test show you all the information you need instead of having to take multiple tests for different markers?


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


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.

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.

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.


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.

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.

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.

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 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.

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.)

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!


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.

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.