What can persistently low c3 (10pt below lab norm) but normal c4 , be due to? dx with sle but sed rate is usually norm w exception of sev flares.

One option: This is a complicated condition so you should seek care from a good Rheumatologist. Within the kidney world a specific type of kidney disease called MPGN- or mebranoprolifferative glomerularnephritis- can be seen with a low C3, normal C4 and also a low CH50. This is diagnosed with a kidney biopsy.

Means nothing: Reference ranges are set so that a few percent of folks fall outside on either side. Sed rates are notoriously inaccurate -- not sensitive, not specific. It's all data that your rheumatologist takes into account in making treatment decisions.