Accuracy of Lymph Node Cytology for Neoplasia Diagnosis

Lymph node cytology is commonly used in companion animal medicine to differentiate causes of lymphadenomegaly, including neoplasia and lymphadenitis. Researchers at the University of California, Davis, recently performed a large-scale, retrospective study to evaluate the diagnostic sensitivity, specificity, and accuracy of lymph node cytology for various types of canine and feline neoplasia.

Analyses

Cytologic samples from 296 dogs and 71 cats obtained over an 11-year period were compared with a histologic gold standard. Histologic lymph node evaluation was performed for each case within 80 days after lymph node cytology, and matched samples taken from the same anatomic location were evaluated when available. For cases in which serial sampling was performed, the cytologic and histologic samples with the shortest time interval were chosen.

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Cytologic samples were obtained via fine-needle aspirate or impression smear and stained with Wright-Giemsa. Histologic samples, stained with hematoxylin and eosin, were obtained via needle core biopsy, surgical biopsy, or necropsy. Board-certified clinical and anatomic pathologists categorized cytologic and histologic samples, respectively, into one of several benign or malignant subgroups. Each cytologic sample’s diagnosis was then classified as true-positive, true-negative, false-positive, or false-negative depending on agreement with the histologic gold standard. Finally, diagnostic accuracy, sensitivity, and specificity values for cytology were determined.

Results

A total of 367 small animal cases included 157 (42.7%) non-neoplastic lesions, 62 (16.9%) lymphoma cases and 148 (40.3%) metastatic neoplasms. The time interval between cytologic and histologic sampling was 0 to 7 days for 202 (55%) cases and 8 to 80 days for 165 (44.9%) cases. The same lymph node was used for cytologic and histologic sampling in 290 (79.0%) cases.

When compared with the histologic gold standard, cytology had a mean diagnostic sensitivity of 66.6% and a mean diagnostic specificity of 91.5% for neoplasia. The diagnostic accuracy (ie, the ability to correctly diagnose a lesion to its subgroup) of cytology was 77.2%. Accuracy, sensitivity, and specificity values were statistically similar regardless of the time interval between cytologic and histologic sampling; however, sensitivity was higher for cytologic samples with a histologic sample from the same lymph node, compared with anatomically mismatched samples.

Among the 367 paired cytologic and histologic cases, 238 (64.8%) showed complete agreement with the same diagnostic subgroup, whereas 43 (11.7%) cases matched only to the level of benign or malignant, and 86 (23.4%) cases disagreed on benign vs. malignant status.

Limitations of Cytology

While the overall likelihood of correctly diagnosing neoplasia via cytology was high, this study found cytology to be poorly sensitive for certain types of neoplasia. False-negative diagnostic rates via cytology were highest for feline mesenteric T-cell lymphoma, metastatic sarcoma, and metastatic mast cell tumor. Low diagnostic sensitivity for these specific neoplasms should be taken into consideration when choosing cytology as a diagnostic tool; however, the authors concluded that lymph node cytology is generally an accurate testing method for primary and metastatic malignant neoplasia in dogs and cats.

Dr. Stilwell is a medical writer and aquatic animal veterinarian in Athens, Georgia. After receiving her DVM from Auburn University, she completed an MS degree in Fisheries and Aquatic Sciences, followed by a PhD degree in Veterinary Medical Sciences, at the University of Florida.