Automated counting of white blood cells in synovial fluid

P. Dieppe and A. Swan

MRC HSRC, Department of Social Medicine, University of Bristol, Canynge Hall, Whiteladies Road, Bristol BS8 2PR, UK

Correspondence to: P. Dieppe. E-mail: p.dieppe{at}bristol.ac.uk

SIR, We were interested to read the article on synovial fluid white count estimations by de Jonge and colleagues [1]. It is clear from data presented in that paper that the total cell count can be accurately estimated by automated methods rather than by more labour-intensive manual counting.

The literature, as de Jonge et al. explain, states that the estimation of total and differential white cell counts in synovial fluid is a valuable and important diagnostic tool in rheumatology. They could find few data on the reliability of different methods for estimating white cell counts; in addition, there appears to be very few empirical data to justify the test being undertaken at all. A recent study completed in our laboratory suggests that it may not add anything to a bedside assessment of joint disease.

We have tested the hypothesis that an experienced rheumatologist can accurately assess the level of inflammation within a joint by clinical examination and gross inspection of synovial fluid alone. We first had a training day for a small group of rheumatologists (consultants and specialist registrars only) at which we agreed upon a four-point scoring system for the clinical assessment of knee joint inflammation (describing features equating to a score of none, mild, moderate or severe synovitis), and a similar scoring system based upon the appearance of synovial fluid. Clinicians then made their assessments of the degree of inflammation both before and after aspirating fluid from knee joints for diagnostic or therapeutic reasons, after which they sent the fluid samples (in lithium heparin bottles) to our laboratory for examination within 24 h of aspiration. In the laboratory we estimated total and differential white cell counts (by manual methods). The laboratory tests were carried out by two independent, experienced technicians who were blind to the clinical data. They grouped fluids into four categories of inflammatory activity on the basis of the total and differential white cell counts.

A total of 53 samples were examined; they were provided by four different rheumatologists. All samples came from the knee joint, but it was not possible to carry out every test on each sample. The age of the patients ranged from 36 to 84 yr and there were equal numbers of men and women. The numbers given a clinical diagnosis of osteoarthritis or of an inflammatory arthritis prior to synovial fluid examination were also similar. The total white cell counts led to the division of these fluids into four categories: non-inflammatory (<105 WBC/ml) n = 5; mild (105–106 WBC/ml) n = 15; moderate (106–107 WBC/ml) n = 20; and severe inflammation (107–108 WBC/ml) n = 10. Similarly, differential cell counts were grouped according to the percentage of polymorphonuclear cells (PMN) as follows: non-inflammatory (<25% PMN) n = 5; mild (25–50% PMN) n = 7; moderate (50–75% PMN) n = 7; and severe (>75% PMN) n = 8. The correlations between the four categories of inflammation based on clinical assessment and those based on white cell counts were high, as shown in Table 1.


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TABLE 1. Correlations between four grades of inflammation (none, mild, moderate or severe) based on clinicians’ assessments before and after synovial fluid aspiration, and those based on total or differential white cell counts (Pearson correlation coefficients)

 
It is apparent that the correlations are better for differential cell counts than for the total counts, and that they are better after gross examination of the fluid by the clinician than from clinical examination of the joint alone.

From these data we conclude that an experienced rheumatologist can accurately assess the degree of inflammation in a joint from clinical examination and the gross appearance of the synovial fluid, and that measuring the total white cell count adds little to the information that these bedside tests provide. Furthermore, if white cell counts are to be measured, it would appear that the differential count is of more value than the total number of white cells in the fluid.

We are grateful to Hanya Amer who was in charge of the laboratory part of this work and to Paul Creamer, Sara Hickey, Iqbal Al-Salem and Clare Emmett for their contributions.

The authors have declared no conflicts of interest.

References

  1. de Jonge R, Brouwer R, Smit M et al. Automated counting of white blood cells in synovial fluid. Rheumatology 2004;43:170–3.[Abstract/Free Full Text]
Accepted 28 May 2004





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