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November 3rd, 2015 | Edition 1, Volume 1

Q. Can you trust an automated counter for your synovial fluid WBC counts?

A. All elevated synovial fluid cell counts should be confirmed by a manual count.

Background: The synovial fluid white blood cell count and the percentage of neutrophils have been reported to be the best traditional tests utilized for the diagnosis of periprosthetic infection.  There have been numerous publications that evaluate the use of automated cell counters for synovial fluid samples with good correlation to manual counts.  However, there have also been reports of automated cell counting errors due to synovial fluid particulate and cellular debris.  Due to the importance and influence of the synovial fluid WBC count in diagnosing PJI, our laboratory evaluated automated WBC counts versus manual cell counts. 
Purpose:  The purpose of this retrospective study is to demonstrate the rate at which automatic counters result in a false-positive synovial fluid WBC count. 

Methods: Synovial fluid samples were pretreated with hyaluronidase and evaluated using the Sysmex 2000 cell counter.  Any synovial fluid with an elevated automated cell count (>3000 cells/uL) was also evaluated using a manual counting chamber to verify the automatic cell count report.  The frequency and magnitude of discrepant results were calculated.  Using the cutoff of 3,000 cells/ul for PJI, the false positive rate due to automatic cell counting was calculated.

Results: 844 synovial fluid samples with a cell count greater than 3000 cells/uL were available for analysis (Table 1). Table 1.For the majority of synovial fluid samples (87.6%), the automatic cell count was within 50% of the manual count.  However, an alarmingly high rate was observed for automatic cell counts that were magnitudes greater than the manual cell count. We found that 5.1% of all automatic cell counts were more than 10-fold greater than the matched manual count, and 8.2% of all automatic cell counts were more than 2-fold greater than the matched manual count.  7.1% of all samples with an automatic cell count over 3000 cells/ul  had and manual count < 3000 cells/ul. 

Conclusions: Although there is a very good correlation between automatic and manual synovial fluid WBC counts, automatic counts frequently report a cell count that is magnitudes higher than the true manual cell count (Fig. 1).  Previous studies have demonstrated that this observation is likely due to particulate matter and cellular debris in the synovial fluid.  Considering the importance of the synovial fluid WBC count in the diagnosis of joint infection, we highly recommend that all automatic cell counts are confirmed with a manual cell count.Fig 1.
Every synovial fluid diagnostic test should be optimized for synovial fluid.

When you order a synovial fluid WBC count, we know that the result may mean the difference between a simple revision or a spacer block.

Particulate matter and cellular debris in a synovial fluid sample can often cause an error on the automatic cell counter, leading to a false-positive synovial fluid WBC count.

A false-positive cell count can be extremely misleading and confusing.

Out of 844 automatic WBC counts >3000 cells/ul that we studied, 7.1% were changed to a normal count of <3000 cells/ul after a reflex manual WBC count was done.

All synovial fluid WBC counts >3000 cells/ul at CD Laboratories are now confirmed by a manual cell count.
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