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Substances found in blood are present at microscopic scale and tend to be invisible to the unaided eye. Why is a whole vial of blood (at ml scale) is required to assess the presence and concentration of substances, given that the modern equipments and testing is becoming more and more sensitive and efficient?

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Can you please specify what kind of blood test you are talking about. And what is "so much blood"? –  Chris Aug 15 at 15:54
    
@Chris I would think that in any blood test you're looking for the presence of microscopic molecules, and thus you'd only need a very small drop of blood. –  user6035 Aug 15 at 16:00
    
In fact a lot of tests are not done using a microscope. And besides that, you can not see molecules in the microscope. You can see cells. Typical amounts for blood tests are 4.5 or 9 ml which is not very much. Enough to handle and prepare the sample and to repeat it, if something goes wrong. –  Chris Aug 15 at 16:07
    
@Chris So is it a question of technology/affordability? Are we capable of obtaining the information from a very small drop of blood? –  user6035 Aug 15 at 16:16
    
That depends on the type of test you want to do. –  Chris Aug 15 at 16:36

1 Answer 1

It might be better to consider the sampling technology, economic and logistical issues with this question as well as the tech behind the tests.

First, some tests still will want a few milliliters of blood - e.g. cell counts for specific cell types. Then there is the need to create and stock many different kinds of sample collection devices and train the collection centers to use them effectively- the industry won't be wanting to completely retool whenever the volume requirement for the assays changes.

Mainly, since the expense of collecting and shipping a few milliliters of blood is pretty much identical to doing so with a few microliters of blood, what has happened is that the tubes are used to do multiple assays with excess included in case an assay has to be redone.

Probably all this blood currently collected is not necessary at all. I expect that the cost of changing all the blood collection systems and protocols is not worth the cost of collecting the exact minimum amount of blood needed. The benefit to the patient or the lab between collecting 3-5 ml of blood and collecting 50 microliters of blood to be shipped off for lab analysis is probably close to zero.

Having more blood than you need does not hurt the lab either should they need to repeat the assay or when you need to use the sample for more than one different test.

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So would it be possible to design a device that runs any blood test on a small drop of blood? –  user6035 Aug 15 at 17:21
    
In many cases it would and people are working on that. Microfluidics is taking the biomed world by storm. That being said, there are potentially thousands of assays that are or will be devised for the market. how many of them will be run out of a smartphone, time will tell. –  shigeta Aug 15 at 20:38
    
Could you expand a little more on how blood tests work and why it's easier to do them with more blood? –  user6035 Aug 15 at 21:44
    
Sorry i'm not being clear. I've added one paragraph above to summarize... –  shigeta Aug 15 at 22:17
    
Could you explain how blood tests actually work? Like do they look at it under a microscope or something? I think if I understood how they test blood it'd help me understand why it's useful to have more of it. –  user6035 Aug 16 at 0:10

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