Situation: The nurse at a Rural Health Unit (RHU) runs the skin clinic of the Leprosy Control Program and follows up the families of clients with leprosy and tuberculosis (TB). A 34-year-old fisherman has three hypopigmented patches with definite loss of sensation. The ulnar nerve at his elbow and the common peroneal nerve at his knee are thickened, and his little finger is weak. A slit-skin smear has not yet been done. How should the nurse classify his leprosy for treatment?

Under the WHO classification used by the DOH, leprosy is multibacillary if there are more than 5 lesions, or nerve involvement, or a positive smear. Any one criterion is enough. Two thickened peripheral nerves with weakness make this case multibacillary even though he has only three patches and no smear result.

The classification of leprosy for treatment purposes follows the WHO field criteria, which determine whether a patient receives the paucibacillary (PB) or multibacillary (MB) regimen. The distinction matters because the two regimens differ in duration and drug combination, and misclassification can lead to undertreatment, relapse, and continued infectivity ^[4].

In this case, the client has three hypopigmented patches with definite sensory loss, thickened ulnar and common peroneal nerves, and weakness of the little finger. The number of skin lesions alone would suggest PB, since the WHO field classification defines PB as 1–5 skin lesions and MB as more than 5 skin lesions ^[3]. However, the presence of thickened peripheral nerves with associated motor weakness changes the classification.

The WHO classification used by the DOH includes three criteria for MB leprosy: more than 5 lesions, nerve involvement, or a positive slit-skin smear. Any single criterion is sufficient to classify the case as MB. Here, two peripheral nerves are thickened and there is demonstrable weakness, which satisfies the nerve involvement criterion even though the smear has not yet been done and the lesion count is below six.

The role of clinical criteria beyond lesion count has been examined in validation studies. One prospective study assessed the sensitivity and specificity of various clinical criteria, including the number of skin lesions, for classifying leprosy into PB and MB ^[1]. Another study explored whether adding criteria such as the number of body areas affected or the size of the largest skin lesion could improve the sensitivity and specificity of the WHO classification ^[2]. These studies highlight that lesion count alone may not capture all cases that require MB treatment, and that nerve involvement is a clinically meaningful indicator of higher bacterial load and more extensive disease.

Key point! Nerve involvement is an independent criterion for MB classification. A thickened peripheral nerve with sensory or motor impairment indicates multibacillary disease regardless of the number of skin patches.

Watch out! Do not wait for the slit-skin smear result before classifying. The smear is confirmatory but not required when clinical criteria for MB are already met. Delaying classification would delay appropriate treatment.

The weakness of the little finger is consistent with ulnar nerve dysfunction, and the thickened common peroneal nerve at the knee further supports peripheral nerve involvement. Two thickened peripheral nerves with associated weakness satisfy the nerve involvement criterion, making this case multibacillary even with only three patches and no smear result.

The rationale for treating this client with the MB regimen is that nerve involvement reflects a higher bacterial burden and a greater risk of disability if undertreated. Misclassification as PB would shorten treatment duration and increase the risk of relapse, which also prolongs the period of infectivity ^[4]. Therefore, the correct classification is multibacillary because a peripheral nerve is involved.