By routine blood test I suppose you mean on that tests hemoglobin and perhaps serum iron levels. This test can tell you if you are suffering from anemia or have an abnormal red blood cell count. That’s about all these test can show. More tests can be done of course but usually they have to be specially ordered by a doctor. All of these are tests of health and not performance indicators. Bear in mind that fitness and thus performance must stand on a base of god health. If you have heath issues such as illness or injury or especially something like low ferritin you will not be able to perform well.
But the simple answer is that no blood test can show performance. MaxVO2 can only be measured but a test in a laboratory where your expired breaths are measured (the apps on most high end HR monitors can not measure maxVO2).
These tests are for monitor health and in that regard they can be very useful. However overtraining, while it is medical situation is so poorly understood that most doctors will not know what to look for. An overtrained athlete is still so much more healthy than 90% of the patients doctors see that a typical response when presented with a case of overtraining is bewilderment: “So, you say you are tired and can go out foe a 290 mile run like normal? Well most people can run one mile so you seem fine to me.”
Use the symptom chart in our book and read the section on overtraining. While its causes are poorly understood. Its symptoms are well documented as is the treatment. Problem is proper diagnosis
I am working on an article on Ferritin and it role in endurance performance. Most docs do not even know to check it and then do not understand what ‘normal’ is for an athlete.