C4A and systemic lupus erythematosus: Using this 98% specificity threshold, the sensitivity of identifying SLE samples was the highest by C4 deposition detection (sum of values in upper quadrants of first two columns: 67%, 66% and 61%) at all three epitope densities tested, exceeding also the combined use of IgG with IgM (sum of values in upper and lower right quadrants of right column: 50%, 55% and 55%, respectively) (Fig 2b).