Castle Biosciences, Inc. (CSTL) Stock

Understand Castle Biosciences, Inc.: how it makes money

Castle Biosciences sells gene-expression diagnostic tests (DecisionDx for melanoma and skin cancers, TissueCypher for Barrett's esophagus) to dermatologists and pathologists on a per-report fee. Revenue splits roughly 60 percent dermatologic and 40 percent non-dermatologic, with TissueCypher as the faster-growing engine.

At 77 percent gross margin on a per-test fee model, each incremental report flows almost entirely to operating profit once fixed R&D and SG&A are absorbed, making volume the sole lever to reach sustained profitability.

Business quality (Weak): High gross margin and 20 percent revenue growth are positive, but the company is still operating at a loss (negative 3 percent operating margin) and the flagship melanoma test is decelerating. The business has not yet proven it can convert its incremental margin into free cash at scale.

Bull case

• TissueCypher Still Early in Adoption: Management notes TissueCypher has substantial remaining physician-conversion runway, meaning the Barrett's esophagus screening line still has several years of volume expansion ahead as more offices adopt the test.
• ADTXT Rate Set at $36.75: The Advanced ADTX reimbursement panel unanimously supported a $36.75 rate, giving the treatment-alignment test a clear, payer-blessed price point that opens a revenue stream beyond pure diagnostic testing.
• Cash Covers Many Quarters: Castle ended Q2 with $266.8 million in cash and marketable securities, more than two and a half years of quarterly revenue, so the current negative 3 percent operating margin does not threaten near-term solvency.

Bear case

• Flagship Melanoma Test Stalling: DecisionDx Melanoma, the original and highest-volume product, grew only 3 percent in the quarter, below typical sequential seasonality, signaling the core addressable market may be maturing faster than the growth model assumes.
• ASP Step-Up Won't Repeat: Management explicitly stated the first-half average selling price improvement will not repeat, capping per-test revenue growth on existing products and leaving volume as the only remaining top-line lever.
• Medicare Reconsideration Creates Overhang: A Medicare reimbursement reconsideration is underway with no defined response timeline, leaving the largest single payer's commitment to the test code in limbo for an indeterminate period.

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