What is the significance of moderate polymorphonuclear (PMN) White Blood Cells (WBCs) on a vaginal wet prep?

Significance of Moderate Polymorphonuclear WBCs on Vaginal Wet Prep

The presence of moderate polymorphonuclear white blood cells (PMNs) on vaginal wet prep is a significant finding that suggests an inflammatory process and supports the diagnosis of infections such as bacterial vaginosis, trichomoniasis, or pelvic inflammatory disease (PID). 1, 2

Clinical Significance

• The presence of WBCs on saline microscopy of vaginal secretions is one of the additional criteria that supports a diagnosis of PID according to CDC guidelines 1, 2
• Elevated numbers of PMNs are found in approximately 30% of women, even when other findings may appear normal 3
• An increased number of white blood cells on wet saline preparation is considered an indication of infection 4
• The finding of moderate PMNs should prompt further investigation for specific infections, as it has limited diagnostic specificity on its own 5

Diagnostic Algorithm

Step 1: Evaluate for PID

• If the patient has uterine/adnexal tenderness or cervical motion tenderness along with moderate PMNs, consider PID diagnosis 1, 2
• Additional supporting criteria for PID include:
  • Oral temperature >101°F (>38.3°C) 1
  • Abnormal cervical or vaginal mucopurulent discharge 1
  • Elevated ESR or CRP 1

Step 2: Evaluate for specific infections

• Check for other microscopic findings on wet mount:
  • Clue cells (bacterial-covered epithelial cells) suggest bacterial vaginosis 4
  • Moving flagellated trichomonads indicate trichomoniasis 4
  • Hyphae or budding yeast indicate candidiasis 4

Step 3: Consider additional testing

• Vaginal pH testing (normal pH should be <4.5; elevated pH is seen with bacterial vaginosis and trichomoniasis) 4
• Whiff test (fishy odor with potassium hydroxide indicates bacterial vaginosis) 4
• Testing for N. gonorrhoeae and C. trachomatis 1, 2

Important Considerations

• Moderate PMNs alone have limited diagnostic utility for specific STIs - sensitivity ranges from 48.2% to 53.9%, and specificity ranges from 67.2% to 68.8% 5
• If the cervical discharge appears normal and no white blood cells are found on wet prep, the diagnosis of PID is unlikely 1
• Trichomoniasis and bacterial vaginosis frequently coexist - approximately 15% of pregnant women with trichomoniasis also have bacterial vaginosis 6
• Standard wet mount microscopy can miss trichomoniasis in 30-50% of cases; more sensitive testing methods include culture, nucleic acid probe, or PCR 4, 7, 8

Common Pitfalls

• Relying solely on vaginal WBC counts for diagnosis of specific infections - additional clinical findings and laboratory tests are necessary 5
• Failing to recognize that elevated PMNs can be present even when other findings appear normal - 54% of women with elevated PMNs may have otherwise normal wet mount findings 3
• Waiting for culture results before initiating treatment - empiric treatment should be started promptly when PID is suspected based on clinical findings 1, 2
• Missing coexisting infections - multiple vaginal infections can occur simultaneously 6

In summary, moderate PMNs on vaginal wet prep represent an inflammatory response that warrants further investigation for specific infections. While not diagnostic on their own, they serve as an important clinical finding that, when combined with other symptoms and test results, guides appropriate diagnosis and treatment.