A 32-year-old woman presents with bilateral anterior uveitis, arthralgia, and erythematous skin lesions on the shins. Chest X-ray shows bilateral hilar lymphadenopathy. Which investigation is most specific for confirming the suspected diagnosis?

Explanation

## Clinical Context The patient presents with a classic triad of bilateral anterior uveitis, erythema nodosum (shin lesions), and arthralgia, combined with bilateral hilar lymphadenopathy on chest X-ray. This constellation is pathognomonic for **sarcoidosis**. ## Why Serum ACE Level? **Key Point:** Serum ACE (angiotensin-converting enzyme) is the most specific biochemical marker for sarcoidosis, particularly when combined with hypercalcemia and elevated 24-hour urinary calcium. **High-Yield:** ACE is produced by activated macrophages in granulomas. Elevated serum ACE (>52 U/L) supports the diagnosis of sarcoidosis and correlates with disease burden and activity [cite:Harrison 21e Ch 335]. **Clinical Pearl:** While ACE is not 100% sensitive, it is highly specific for sarcoidosis when elevated in the appropriate clinical context (uveitis + systemic features + hilar LAD). ## Diagnostic Approach for Sarcoidosis | Investigation | Role in Sarcoidosis | Sensitivity/Specificity | |---|---|---| | Serum ACE | Specific marker; reflects granuloma burden | 60% sensitive, 90% specific | | Chest X-ray | Establishes pulmonary involvement | High sensitivity for hilar LAD | | Biopsy (conjunctival/lacrimal gland) | Confirms non-caseating granulomas | Gold standard but invasive | | 24-hour urinary calcium | Detects hypercalciuria (granuloma-mediated) | Supports diagnosis | | Mantoux test | Usually negative in sarcoidosis | Helps exclude TB | **Tip:** The diagnosis of sarcoidosis is clinical + radiological + biochemical. ACE level is the single most specific *laboratory* test; biopsy is the gold standard but not always needed if clinical + radiological + ACE findings align. ## Why Not the Other Options? - **Mantoux test:** Typically negative or weakly positive in sarcoidosis (due to anergy); used to *exclude* TB, not confirm sarcoidosis. - **HLA-B27:** Associated with HLA-B27–positive anterior uveitis (ankylosing spondylitis, reactive arthritis), not sarcoidosis. - **Chest CT:** Provides anatomical detail but does not confirm the diagnosis; it is a staging tool, not a diagnostic test.