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In vitro study of amniotic fluid gram stain: effect of centrifugation
D W Gauthier1, W Torres, W J Meyer
1Department of Obstetrics and Gynecology Division of Maternal-Fetal Medicine University of Illinois College of Medicine 820 South Wood Street, M/C 808, Room 250, Chicago, IL 60612-7313, USA.
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 < or =10(4) cfu/ml (P < 0.01). At concentrations > or =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.
Insights
Centrifugation significantly enhances Gram stain detection of bacteria in amniotic fluid (AF) at low concentrations (≤10^4 cfu/ml). This method improves the identification of intraamniotic infections when bacterial levels are minimal.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Obstetrics
Background:
- Intraamniotic infection is a critical complication in preterm labor and preterm premature rupture of membranes.
- Gram stain of amniotic fluid (AF) is a primary diagnostic tool for detecting intraamniotic infection.
- The sensitivity of AF Gram stain in detecting low bacterial loads remains a challenge.
Purpose of the Study:
- To evaluate whether centrifugation enhances the diagnostic accuracy of Gram stains for detecting bacteria in amniotic fluid.
- To determine the impact of centrifugation on bacterial detection rates at various concentrations.
Main Methods:
- Amniotic fluid samples from patients with preterm labor or preterm premature rupture of membranes were used.
- Bacterial suspensions were prepared and diluted in AF to concentrations ranging from 10^3 to 10^6 colony forming units (cfu)/ml.
- AF samples were divided, with one portion centrifuged and the other uncentrifuged, followed by Gram staining and blinded interpretation.
Main Results:
- Centrifugation significantly improved bacterial detection by Gram stain at concentrations of 10^4 cfu/ml or lower (P < 0.01).
- No significant improvement in bacterial detection was observed with centrifugation at concentrations of 10^5 cfu/ml or higher.
- Fisher's exact test was employed to compare detection rates between centrifuged and uncentrifuged samples.
Conclusions:
- Centrifugation of amniotic fluid increases the sensitivity of Gram stain for detecting low levels of bacteria.
- This pre-processing step can aid in the earlier and more accurate diagnosis of intraamniotic infections.
