Common TB Drug, Rare Blood Reaction: Case Report Links Ethambutol to Platelet Drop

Ethambutol is widely used as a first-line agent in the treatment of tuberculosis and is also a key component of therapy for atypical mycobacterial infections. Compared with other anti-tuberculosis medications, it is generally associated with a lower rate of adverse drug reactions. Optic neuropathy remains the most recognized toxicity, while hematological complications are considered exceptionally uncommon. This case adds to the limited evidence suggesting that ethambutol can, in rare instances, lead to significant platelet abnormalities.

The authors described the case of a 41-year-old man who was undergoing treatment for cervical lymph node tuberculosis. Shortly after starting standard antituberculous therapy, the patient developed isolated thrombocytopenia. A marked decline in platelet count was detected 10 days after treatment initiation, raising concerns about a potential drug-related cause. At that time, there were no clinical signs of bleeding, and no alternative causes of thrombocytopenia were identified through clinical assessment or laboratory evaluation.

Given that drug susceptibility testing showed no resistance to other first-line tuberculosis medications, ethambutol was discontinued while the remaining agents were continued. Despite stopping the drug, the platelet count continued to fall briefly, reaching a nadir of 72×10⁹/L approximately 48 hours after discontinuation, following nearly three weeks of therapy. Importantly, platelet levels began to recover rapidly thereafter, returning toward normal within five days without the need for additional interventions.

To assess the likelihood of a causal relationship, the investigators carefully reconstructed the treatment timeline and applied the Naranjo adverse drug reaction probability scale. Even without reintroducing ethambutol, the patient’s Naranjo score was calculated as +6, indicating a probable association between ethambutol exposure and thrombocytopenia. The temporal pattern of onset and recovery strongly supported this conclusion.

The authors also compared this case with previously published reports and noted striking similarities. To date, only two other cases of ethambutol-associated thrombocytopenia have been documented in the literature. Like the present case, those reports described platelet decline occurring within the first two weeks of therapy and rapid resolution following drug withdrawal.

The findings reinforce the importance of considering ethambutol as a potential cause of unexplained thrombocytopenia in patients receiving tuberculosis treatment, even though the reaction is rare. Early recognition is crucial, as prompt discontinuation of the drug appears sufficient to reverse the condition and prevent serious complications.

While ethambutol remains a cornerstone of tuberculosis therapy, this case highlights the value of close clinical and laboratory monitoring during treatment. The authors conclude that clinicians should maintain a high index of suspicion for uncommon adverse effects, ensuring timely intervention and safe continuation of effective anti-tuberculosis regimens.

Reference:

Françoise U, Henry K, Sidibe M, Lalande V, Cholle C, Le Turnier P. Ethambutol-Associated Thrombocytopenia: A Rare Case Report of Drug-Induced Platelet Decline in Tuberculosis Treatment. Am J Case Rep. 2025 Dec 20;26:e945090. doi: 10.12659/AJCR.945090. PMID: 41420853.