What if no synovial fluid is obtained (a “dry tap”)?
Even if no fluid is aspirated into the syringe, frequently one or two drops of fluid and/or blood can be found within the needle and its hub. This amount is sufficient for culture, in which case the syringe with a capped needle should be submitted to the microbiology laboratory. If one extra drop can be spared, it can be placed on a microscope slide with a coverslip for estimated cell count (1 white blood cell [WBC]/40 × objective = 500 WBCs) and polarized microscopy. When microscopy is completed, the coverslip can be removed and the specimen may then serve as a smear for Gram stain. The specimen remaining on the coverslip may be an adequate smear on which to perform a Wright stain, allowing determination of leukocyte differential. Thus, two drops of fluid can yield the same important diagnostic information as that obtained from a larger specimen, with the exception of a leukocyte count. The lesson to be learned from this is that when a “dry tap” is encountered, the needle and syringe should not be reflexively discarded.
