Methods for diagnosing systemic lupus erythematosus

1. A method for treating systemic lupus erythematosus in a human subject in need thereof, wherein the human subject is negative for systemic lupus erythematosus based on a level of double-stranded DNA antibody; the method comprising:

(a) measuring a level of erythrocyte-bound complement component C4d (EC4d) marker in a blood sample obtained from the patient;

(b) measuring a level of B-lymphocyte-bound C4d complement component (BC4d) marker in the blood sample;

(c) measuring a level of anti-nuclear antibody (ANA) in the blood sample;

(d) determining a 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

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

(f) treating the human subject from step (e) 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 (a) comprises measuring the level of EC4d marker by enzyme-linked immunosorbent assay, indirect immunofluorescence, radioimmunoassay, or flow cytometry; and step (b) comprises measuring the level of BC4d marker by enzyme-linked immunosorbent assay, indirect immunofluorescence, radioimmunoassay, or flow cytometry.

3. The method of claim 1 , wherein step (c) comprises measuring the level of ANA in the blood sample by enzyme-linked immunosorbent assay, indirect immunofluorescence, radioimmunoassay, or flow cytometry.

4. The method of claim 1 , wherein log in steps (d)(1) and (d)(2) is natural log.

5. The method of claim 1 , wherein step (d)(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.

6. The method of claim 1 , wherein measuring the level of EC4d marker comprises measuring the level of EC4d on the surface of erythrocytes.

7. The method of claim 1 , wherein measuring the level of BC4d marker comprises measuring the level of BC4d on the surface of B lymphocytes.

8. The method of claim 1 , wherein the human subject has a level of EC4d marker that is less than 6 standard deviations above the mean EC4d marker among non-lupus patients.

9. The method of claim 1 , wherein the human subject has a level of BC4d marker that is less than 6 standard deviations above the mean BC4d marker among non-lupus patients.