Laboratory Studies
The diagnosis of chancroid based solely on the ulcer's appearance is accurate only in 30-50% of cases. In areas of high prevalence, the accuracy for a clinical diagnosis of chancroid is as high as 80%; however, in areas where the disease is less common, the clinical basis for diagnosing chancroid leads to overdiagnosis. Considerable overlap exists between the major causes of gential ulcer disease (GUD): herpes simplex virus (HSV), syphilis, chancroid, and, often, co-infection with 2 diseases at the same time. Co-infection with syphilis or HSV occurs in as many as 10% of patients. Realizing that no etiology can be found in 25-50% of all cases of GUD is important.
Gram staining may show gram-negative coccobacilli singly, in clusters, or in various morphological forms described as "schools of fish," "railroad tracks," or "fingerprints." The organisms are visualized extracellularly more often than intracellularly and tend to occur in close proximity to polymorphonuclear leukocytes. Gram staining of an ulcer specimen is not highly specific or sensitive. Interpretation is hampered by polymicrobial contamination. However, slides with gram-negative coccobacilli in parallel rows or in a clustered school-of-fish pattern have been reported to have a sensitivity of 62% and a specificity of 99% for chancroid.
Direct microscopy should not be used in the routine diagnosis of chancroid.
Culture is now the accepted standard for chancroid diagnosis in most areas, but even in an experienced laboratory, it is only 60-80% sensitive. Numerous selective artificial media have been developed, but 2 media are commonly used. Nairobi medium consists of a biplate of (1) gonococcal agar base with 2% bovine hemoglobin and 5% fetal calf serum and (2) Mueller-Hinton agar with 5% chocolatized horse blood. Both media contain vancomycin (3 μg/mL) and 1% CVA (ie, cephalothin, vancomycin, and amphotericin B) enrichment. The use of both media together increases the yield of positive cultures. Most H ducreyi organisms are resistant to vancomycin, but some strains are inhibited by its presence. Therefore, negative culture results in the setting of high suspicion should prompt screening for vancomycin-sensitive organisms. [37]
A simple, inexpensive medium containing gonococcal agar base supplemented with 5% Fildes' extract and unchocolatized horse blood or a medium containing 0.2% activated charcoal instead of fetal calf serum is a suitable alternative for diagnostic purposes in resource-poor countries. H ducreyi is a fastidious organism. Patients' specimens must either be plated out directly on an appropriate culture medium or sent to the microbiology laboratory for culture as soon as possible. No widely available transport medium exists. [38] Studies have shown cultures to be less sensitive in women than in men.
Purulent material aspirated from intact buboes is almost always sterile.
Polymerase chain reaction (PCR) amplification is replacing culture as the diagnostic test of choice in some major medical centers. PCR amplification using a variety of primers may provide a useful alternative to culture for the detection of H ducreyi. Although PCR assays perform well on samples prepared from H ducreyi cultures, they are less sensitive when used to test genital ulcer specimens. [39]
A multiplex PCR assay has been developed for the simultaneous amplification of DNA targets from H ducreyi, T pallidum, and HSV types 1 and 2 and appears more sensitive than standard diagnostic tests for the detection of these etiologic agents in genital ulcer specimens. The multiplex PCR assay has been developed but is not yet commercially available. [40, 41]
Monoclonal antibody–based technology has the potential to provide a simple, inexpensive, rapid, and sensitive means of detecting H ducreyi in genital ulcer specimens, but no assay kits are commercially available. [39]
Serology has limited usefulness in the routine diagnosis of chancroid infection.
Tissue biopsy is not a recommended diagnostic method for chancroid but may be useful as a means to exclude malignancy in nonhealing or atypical ulcers.
Serologic tests for syphilis include the Venereal Disease Research Laboratory (VDRL) test, rapid plasma reagin (RPR) test, and a darkfield examination.
Tests for HSV include a Tzanck smear, direct fluorescence microscopy, and culture.
HIV testing should be performed in all patients who have genital ulcers caused by H ducreyi.
Patients should be retested for syphilis and HIV 3 months after the diagnosis of chancroid if the initial test results were negative.
Consider tests for other sexually transmitted diseases (STDs), including hepatitis B, Chlamydia trachomatis infection, and gonorrhea.
As PCR and other techniques become available, cultures may be indicated to obtain a specimen for antibiotic sensitivity testing.
Histologic Findings
Histologic features from a biopsy sample of a chancroid ulcer overlap significantly with those of syphilitic chancres. Lesions show 3 zones. The most superficial zone is narrow and consists of neutrophils, fibrin, erythrocytes, and necrotic tissue. The second zone is wider and contains newly formed blood vessels with marked endothelial cell proliferation. The lumina of many of these vessels are occluded, leading to thrombosis. The deepest zone is composed of a dense infiltrate of plasma cells and lymphoid cells. H ducreyi bacilli are seldom demonstrable on biopsy samples. Tissue biopsy is not a recommended diagnostic method for chancroid but may be useful as a means to exclude malignancy in nonhealing or atypical ulcers.
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Chancroid usually starts as a small papule that rapidly becomes pustular and eventually ulcerates. The ulcer enlarges, develops ragged undermined borders, and is surrounded by a rim of erythema. Unlike syphilis, lesions are tender and the border of the ulcer is not indurated. Courtesy of Hon Pak, MD.
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This patient shows the characteristic lesions of chancroid. The bubo on the right side drained spontaneously. The bubo in the left inguinal canal required needle aspiration.
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Close-up view of chancroid ulcers.
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Large fluctuant buboes should be drained with the patient under local anesthesia and a large-gauge needle inserted through surrounding healthy skin. The insertion site should be superior or lateral to the bubo to prevent chronic drainage from the site.
