If blood tests come back normal, does that mean you don't have lymphoma? how is lymphoma truly diagnosed?

Biopsy: Blood tests are frequent normal in many lymphomas unless the extent of disease is quite advanced and affects the function of the bone marrow, for example. The gold standard and only way to truly diagnose a lymphoma is with a biopsy of an affected lymph node or lesion. A pathologist will examine it under the microscope and special tests can be performed on the tissue obtained.

Lymphoma.: Lymphomas are a diverse group of diseases. Usually diagnosed by clinical suspicion followed by confirmatory labs and imaging. Normal labs do not rule out the disease but if there is nothing to suggest the disease then normal labs almost certainly excludes it. If there is a nodule or suspicious lesion a biopsy is in order.