Methods for diagnosing systemic lupus erythematosus

1. A method for treating systemic lupus erythematosus in a subject in need thereof, the method comprising:

(a) measuring a level of double stranded DNA antibody in a blood sample from the subject with an enzyme-linked immunosorbent assay, and determining that the subject is negative for double stranded DNA antibody;

(b) measuring a level of erythrocyte-bound complement component C4d (EC4d) protein in the blood sample with fluorescence-activated cell sorting;

(c) measuring a level of B-lymphocyte-bound C4d complement component (BC4d) protein in the blood sample with florescence-activated cell sorting;

(d) measuring a level of anti-nuclear antibody (ANA) in the blood sample with an enzyme-linked immunosorbent assay or fluorescence-activated cell sorting;

(e) determining the systemic lupus erythematosus risk score comprising the steps of:

(i) log normalizing the level of EC4d to produce a log normalized level of EC4d, and multiplying the log normalized level of EC4d by a predetermined weighting coefficient to produce a weighted score for EC4d,

(ii) log normalizing the level of BC4d to produce a log normalized level of BC4d, and multiplying the log normalized level of BC4d by a predetermined weighting coefficient to produce a weighted score for BC4d,

(iii) multiplying the level of ANA by a predetermined weighting coefficient to produce a weighted score for ANA, and

(iv) summing the weighted score for each of EC4d, BC4d, and ANA, thereby determining the systemic lupus erythematosus risk score; and

(f) determining that the subject has systemic lupus erythematosus based on the systemic lupus erythematosus risk score; and

(g) treating the subject from step (f) that has systemic lupus erythematosus based on the systemic lupus erythematosus risk score with an effective amount of cyclophosphamide, a corticosteroid, mycophenolate, methotrexate, azathioprine, leflunomide, belimumab, or hydroxychloroquine.

2. The method of claim 1 , wherein step (e)(iii) comprises multiplying a number associated with a cutoff value for the level of ANA by a predetermined weighting coefficient to produce a weighted score for ANA.