How could a doctor confirm that you have osteoarthritis when your blood tests come back normal?

With x-rays or MRI.: There are blood tests for rheumatoid arthritis but none for osteoarthritis. The first step is to obtain weight bearing x-rays of the knee. If these are unremarkable, then a MRI scan provides more detail and can demonstrate mild osteoarthritis not yet apparent on x-rays. If there is no sign of osteoarthritis on x-rays or mri, it is unlikely that it is present.

X-ray exam: Osteoarthritis is clearly visible on x-ray with joint space narrowing due to cartilage degeneration.

Tests are normal : Blood tests are normal if you have osteoarthritis. The cartilage surface of a joint cna wear down with time and use. Blood tests rule out inflammatory conditions like rheumatoid arthritis