If serum protein electrophoresis shows no evidence of a monoclonal protein, what should be the next step?

When serum protein electrophoresis does not show evidence of a monoclonal protein, the next appropriate step is to report the results and request a urine sample. This approach is essential for further evaluation of potential disorders such as multiple myeloma or other plasma cell dyscrasias, where a monoclonal protein might be present in the urine even if it is not detected in the serum.

The presence of Bence Jones proteins (light chains of immunoglobulins) can sometimes occur in these conditions and may not be reflected in serum tests. By requesting a urine sample, clinicians can utilize additional tests, such as urinalysis or urine protein electrophoresis, to detect these light chains, thus providing a more comprehensive assessment of the patient's plasma cell activity.

In context, quantitative immunoglobulin tests might be helpful but are not the immediate next step after a negative electrophoretic finding. IFE, or immunofixation electrophoresis, is also beneficial but is ideally carried out after urine sampling to clarify whether light chains are present. Hence, ensuring a urine sample is collected provides a pathway for deeper investigation into potential underlying conditions.