Diagnostic Confirmation

The definitive diagnosis of PF is made by a corroborative history and clinical signs in addition to suggestive cytologic and histologic findings. The diagnosis should be confirmed by ruling out other differential diagnoses as well as by finding negative bacterial and fungal cultures. Diagnostic cytology is best obtained from lesions such as an intact pustule or exudate on the undersurface of a crust. Direct impression smears from pustules, undercrusts, or very recent erosions provide one reliable method from which the cell types suggestive of PF are most likely to be found. To collect interpretable specimens for cytology, a pustule should be gently lifted toward the glass slide to enhance sample collection. The glass microscope slide is then pressed onto the site or pulled gently from one end to the other over the lesion. To obtain a cellular sample, it is important to apply pressure directly over the target lesion. Using digital pressure from the index finger or thumb easily accomplishes this while minimizing the chance of breaking the glass slide during the impression smear. Enough pressure has been applied if there is an impression of cell debris on the unstained slide. In addition, crusts can be lifted to reveal the moist undersurface. Collect the moist exudate by gently pressing the undersurface of the crusts several times onto the slide. Alternatively, in a cooperative patient, a 25-gauge needle can be used to gently “lift the lid off” a pustule to expel its contents, allowing an impression smear to be made from the suppurative exudate. The slide is then stained with a commercially available cytology stain (e.g., modified Wright stain such as Diff Quick), and then gently rinsed with water.