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
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.
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