Which investigation is primarily diagnostic for multiple myeloma?

A bone marrow biopsy is the primary diagnostic investigation for multiple myeloma because it allows for direct examination of plasma cells within the bone marrow. In multiple myeloma, there is a pathological proliferation of these malignant plasma cells, which can be identified through this biopsy. The procedure involves taking a sample of bone marrow, typically from the hip or sternum, and evaluating it under a microscope to assess for the presence and quantity of abnormal plasma cells, which is a hallmark of the disease.

While other investigations such as a complete blood count can provide supportive information—like anemia or changes in white blood cells— and urine analysis can show elevated levels of monoclonal light chains (Bence Jones proteins), they do not confirm the diagnosis as definitively as a bone marrow biopsy does. Similarly, serum creatinine levels are important for assessing kidney function, which can be affected in multiple myeloma, but they are not diagnostic of the condition itself. Hence, the bone marrow biopsy is essential in establishing the diagnosis of multiple myeloma.