In Vitro Study of Amniotic Fluid Gram Stain: Effect of Centrifugation

Objective: Gram stain of amniotic fluid (AF) is used to detect intraamniotic infection. The purpose of this study was to determine if centrifugation improved the ability of AF Gram stain to detect bacteria. Methods: AF obtained by amniocentesis from patients with preterm labor (PTL) or preterm premature rupture of membranes (PPROM) was pooled. Individual AF samples as well as the pooled sample had a negative Gram stain for microorganisms or white blood cells (WBCs) and negative cultures. With pure bacterial cultures, a suspension equivalent to a 0.5 McFarland turbidity standard was prepared and then serially diluted in the AF to either 10(6), 10(5), 10(4), or 10(3) colony forming units (cfu)/ml. Each sample was divided into 2 equal portions, with 1 undergoing centrifugation. The Gram stains were interpreted by technologists in the clinical microbiology laboratory in a blinded fashion. Fisher's exact test was used to compare the bacterial detection rate in centrifuged vs. uncentrifuged AF samples at each concentration. Results: Centrifugation of AF significantly improved the ability of the Gram stain to detect bacteria at bacterial concentrations ≤10(4) cfu/ml (P < 0.01). At concentrations ≥10(5) cfu/ml, centrifugation did not improve the ability of the Gram stain to dtect bacteria. Conclusions: At low bacterial concentrations, centrifugation of AF increases the bacterial detection rate of AF Gram stain.