To the Editors:

We read with great interest the recent article by Caspi et al (1) on synovial fluid levels of anti–cyclic citrullinated peptide (anti-CCP) antibodies. The evidence of increased levels of anti-CCP antibodies in synovial fluid of patients with rheumatoid arthritis (RA) in comparison with other arthritides (1, 2), showing a sensitivity of 80% and a specificity of 78%, could sustain the clinical role of anti-CCP synovial fluid determination, mainly in RA patients seronegative for anti-CCP. Hence, we studied 29 patients with RA classified according to the American College of Rheumatology criteria (3), with or without anti-CCP antibodies in sera, and compared them with 15 osteoarthritis patients presenting with knee joint effusion. In all patients, the level of anti-CCP was determined by enzyme-linked immunosorbent assay (Quanta Lite CCP IgG ELISA, INOVA Diagnostics, San Diego, CA) in synovial fluid and serum. Routine laboratory tests included the analysis of synovial fluid, the erythrocyte sedimentation rate, and determination of C-reactive protein levels. Rheumatoid factor (RF) (IU/ml) and IgG (gm/liter) were determined using the N Latex RF Kit and BN 100 nephelometer (Dade Behring, Marburg, Germany). The main demographic, clinical, and laboratory features of RA patients, with or without anti-CCP antibodies in serum, are shown in Table 1.

Table 1. Main demographic, clinical, and laboratory features of patients with rheumatoid arthritis with or without anti-CCP antibodies in serum*
CharacteristicsNegative serum anti-CCPPositive serum anti-CCP
  • *

    Values are the median (25th–75th percentile) unless otherwise indicated. Anti-CCP = anti–cyclic citrullinated peptide; DMARDs = disease-modifying antirheumatic drugs; ESR = erythrocyte sedimentation rate; CRP = C-reactive protein; SF = synovial fluid; WBC = white blood cells; PMN = polymorphonuclear; RF = rheumatoid factor.

  • P < 0.005

  • P < 0.00005

  • §

    P < 0.000001

Patients, no.1316
Age, mean (range) years49 (26–67)56 (31–76)
Male:female, no.3:105:11
Disease duration, mean (range) months131 (6–588)94 (12–360)
Disease onset, mean (range) years38 (18–58)49 (27–72)
DMARDs, no. (%)10 (77)14 (87)
Corticosteroids, no. (%)12 (92)14 (87)
ESR, mm/hour30 (17–50)40 (21–55)
CRP, mg/dl1.2 (0.5–2.4)1.5 (0.3–4.8)
SF low viscosity, no. (%)12 (92)14 (84)
SF mucin clot, no. (%)  
 Good2 (15.4)3 (18.75)
 Fair7 (53.8)9 (56.25)
 Poor4 (30.8)4 (25.0)
SF WBC, /mm38,000 (6,000–13,500)8,000 (6,600–18,075)
SF PMN cells, /mm36,000 (1,520–11,475)6,000 (4,175–14,815)
SF mononuclear cells, /mm32,560 (1,800–4,800)2,210 (1,900–3,200)
Positive serum RF, no. (%)2 (15.4)12 (75)
SF anti-CCP, units6 (4–6.3)110 (49–282)
Serum anti-CCP, units5 (5–6)186 (55–282)§
SF IgG, gm/liter6.61 (6.2–8.8)5.77 (4.52–7.46)
Serum IgG, gm/liter14.1 (11.90–16.49)10.98 (8.58–12.99)
SF IgG anti-CCP, unit/total IgG gm/liter0.7 (0.6–1.3)24.4 (6.5–38.5)

Only one RA patient with no serum anti-CCP antibodies was weakly positive for anti-CCP in synovial fluid. Nevertheless, the different levels of synovial fluid immunoglobulins, including autoantibodies, among patients with arthritis and healthy subjects must take into account the potential factors of protein concentrations in synovial fluid, including microvascular permeability, interstitial properties, mode of transport, characteristics of proteins (i.e., molecular weight or radius), synovial metabolism, and vascularity and lymphatic drainage (4). Thus, the adequate interpretation of synovial fluid protein levels might be the expression of autoantibody concentrations corrected for the total amount of the corresponding class-specific immunoglobulin (2), i.e., IgG anti-CCP (unit)/total IgG (gm/liter). In our study, this approach showed that RA patients with serum anti-CCP had synovial fluid anti-CCP/IgG ratio higher than that in RA patients with no serum anti-CCP antibodies (see Table 1). Moreover, in this last group 2 patients exhibited anti-CCP/IgG ratio values above the cut-off (2.22 units/gm/liter) calculated as the value for the upper 95th percentile in synovial fluid of osteoarthritis patients.

In conclusion, our results suggest that the synovial fluid determination of anti-CCP antibodies should be reserved for selected cases (i.e., seronegative patients for these autoantibodies) taking into account the correction for the total amount of the corresponding immunoglobulin.

Antonio Spadaro Prof*, Valeria Riccieri MD*, Rossana Scrivo MD*, Cristiano Alessandri MD*, Guido Valesini Prof*, * University of Rome, Rome, Italy.