How to Perform and Interpret Findings from a Low-Volume Uterine Flush

1. Introduction

Bacterial uterine infections occur in 25% to 60% of barren mares and inflict major losses on the equine breeding industry. An accurate diagnosis based on history, clinical findings, uterine culture, and cytology, and, in some cases, histology, is mandatory if mares are to be treated successfully. Correct interpretation of microbiological and cytological findings requires consideration of possible false-positive and false-negative culture results. False-positive cultures have been associated with contamination of the culture instrument, whereas false-negative cultures have been associated with inadequate sampling of the endometrium. Nielsen (2005) has shown that only 38 of 84 mares (45%) with bacteria isolated from the surface of an endometrial biopsy had bacteria isolated from a uterine culture swab.1 More recently, he reported a higher proportion of sterile, cytology-positive cases when endometrial samples were obtained by swabs than by endometrial biopsies (148/401, 37% versus 12/237, 5%, respectively; P < 0.0001).²  Infections may be missed because bacteria may be located focally, deep within the uterine body or uterine horns, and endometrial swabs contact only a 1- to 2-cm area of endometrium cranial to the cervix. Not all bacteria induce a strong neutrophilic uterine response with subsequent fluid production, further limiting accurate swab culture results.^2–4  Uterine flushes are an alternative method for obtaining uterine samples. They have been used for many years in endometritis research to obtain microbiological and cytological specimens from the mare’s endometrium.^5–9  We recently investigated a low-volume flush technique in a large group of infertile mares in clinical practice and found it to be a rapid, accurate method for identifying mares with chronic endometritis.¹⁰  In this report, we describe the technique of how to interpret findings and include previously published results from 401 uterine flushes. 

2. Materials and Methods

A low-volume uterine flush can be performed during estrus or diestrus. Performing the flush during diestrus frequently results in recovery of a larger amount of fluid because the infused fluid can be trapped among the prominent, edematous endometrial folds during estrus. If the sample is collected during diestrus, it is recommended that the mare be given prostaglandin so that she returns to estrus within a few days. Before the procedure, the rectum should be evacuated, the tail wrapped and deviated laterally, and the perineum scrubbed, rinsed, and dried. Ten to 20 cm of a sterile uterine catheter is passed per vaginum into the uterus and up into a uterine horn by an examiner whose arm is covered by a sterile sleeve. Commercially obtained uterine lavage tubes or medical grade silicone tubing can be used (Fig. 1). If the latter is used, catheters should be cut to 60- to 80-cm lengths.^a Two to three holes should be made about 3 cm from the end of the catheter with a scalpel blade and the end beveled to smooth the roughened edges. Medical grade silicone tubing can be autoclaved. Sterile saline is infused into the uterus by attaching either a 60-mL catheter-tip syringe containing 60 mL of saline (Fig. 2) or a 150-mL bag of sterile saline to the end of the catheter (Fig. 1). The uterus is then manipulated by transrectal palpation for a minimum of 30 seconds to distribute the sterile saline throughout the uterine lumen. The uterine horn containing the catheter tip is cradled transrectally by the veterinarian’s hand and the saline is either drained into a sterile 50-mL conical tubeb by gravity flow or the fluid is allowed to drain back into the 150-mL bag (Fig. 3). If one is working without an assistant, extension tubing can be attached between the uterine lavage catheter and 150-mL saline bag for easier manipulation. However, the extra length of the tubing creates negative pressure, impeding fluid recovery. The efflux recovered in the bag is transferred into a 50-mL conical tube. The volume of fluid recovered is recorded. It must exceed the fluid volume held within the uterine catheter for the sample to be considered an accurate representation of the uterine luminal contents. Clarity of the fluid is recorded as being cloudy, clear, or containing mucus strains (Fig. 4). Degree of opacity or thickness and amount of mucus strains can also be noted using a numerical system from 0 to 3, with 0 being clear. Mucus strains are best viewed by rotating the tube while holding it up to the light. The precipitant is allowed to settle, or the sample can be centrifuged at 400g for 10 minutes and all but 5 mL of supernatant is poured off. Two sterile cotton-tipped swabs are placed into the pellet at the bottom of the tube. One is used for uterine culture and the second is used for a cytological specimen. Samples must be processed within 8 hours because the saline does not preserve the bacteria.
 

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