Do medical laboratories determine accuracies for their blood tests that have numerical results (as opposed to specificities and sensitivities for tests with binary results)? If so, where can I find these accuracies? If not, why not?
NOTE: I cannot give you any references in english, since the documentation I have to backup these claims is in Portuguese.
Do medical laboratories determine accuracies for their blood tests that have numerical results?
In order for a lab to be certified (in my country at least, but I reckon this is probably true for other countries), periodic calibrations and validations of the lab equipment and methodology are required.
Different types of tests require different methods of validation but they usually fall into two (very) broad categories:
Validation against a known substance/quantity;
Example: To check if the chlorine reader is working properly, you test the machine against a sealed vial containing a known concentration of chlorine.
Blind checks against the results of another certified labs.
Example: Blood cell count (automatic or manual (by a human, on a microscope)). A blood sample is divided evenly into two vials and one is sent to another certified lab. Usually several measures of the same sub sample are performed, in order to reduce sampling error.
For a lab to reach the maximum certification level, both types of tests are performed several times for each lab test.
If so, where can I find these accuracies? If not, why not?
Regarding the "accuracy", it depends on many factors (method of collecting samples, method of transportation of sample, the time bewteen collection and test run, etc...).
But for automatic tests, but the most important is the "machine" used to test it. The machine's manual usually gives you the value of the error or the confidence interval.
So, in order to get a pretty good estimate of the "accuracy" of the lab test, you only need to know the "machine" used and you can google search for the manual or brochure to find that out. example. Or, failing that, call the manufacturer and ask for that value.
If your country or state has a certification level, a quicker way might be through the certification level of the lab. Each level has a "tolerance" value, that is, the maximum acceptable deviation from the expected value.
Also, for tests that rely heavily on humans, the "tolerance" value is the only way you can estimate the "accuracy" of the test.