CSTL vs ARDX

For growth investors, Ardelyx, Inc.

(ARDX) is the stronger pick with 22. 1% revenue growth year-over-year, versus 3. 7% for Castle Biosciences, Inc. (CSTL). Analysts rate Castle Biosciences, Inc. (CSTL) a "Buy" — based on 11 analyst ratings — the highest consensus in this comparison. The "better buy" depends entirely on your goals: growth investors should weight revenue trajectory, value investors should weight P/E and PEG, and income investors should weight dividend yield and streak.

Over the past 5 years, Ardelyx, Inc.

(ARDX) delivered a total return of +313. 0%, compared to -68. 1% for Castle Biosciences, Inc. (CSTL). Over 10 years, the gap is even starker: ARDX returned +263. 5% versus CSTL's -10. 1%. Past returns do not guarantee future results, and the stock with the higher historical return may already have its best growth priced in.

By beta (market sensitivity over 5 years), Ardelyx, Inc.

(ARDX) is the lower-risk stock at 0. 87β versus Castle Biosciences, Inc. 's 0. 88β — meaning CSTL is approximately 2% more volatile than ARDX relative to the S&P 500. On balance sheet safety, Castle Biosciences, Inc. (CSTL) carries a lower debt/equity ratio of 8% versus 127% for Ardelyx, Inc. — giving it more financial flexibility in a downturn.

By revenue growth (latest reported year), Ardelyx, Inc.

(ARDX) is pulling ahead at 22. 1% versus 3. 7% for Castle Biosciences, Inc. (CSTL). On earnings-per-share growth, the picture is similar: Ardelyx, Inc. grew EPS -52. 9% year-over-year, compared to -233. 9% for Castle Biosciences, Inc.. Over a 3-year CAGR, ARDX leads at 98. 4% annualised revenue growth. Higher growth typically commands a higher valuation multiple — check whether the premium P/E or P/S is justified by the growth rate using the PEG ratio.

Castle Biosciences, Inc.

(CSTL) is the more profitable company, earning -7. 0% net margin versus -15. 1% for Ardelyx, Inc. — meaning it keeps -7. 0% of every revenue dollar as bottom-line profit. Operating margin tells a similar story: ARDX leads at -10. 1% versus -12. 4% for CSTL. At the gross margin level — before operating expenses — ARDX leads at 89. 5%, reflecting greater pricing power or product mix advantage. Stronger margins indicate durable pricing power, lower cost of revenue, or higher mix of software/services. They are one of the clearest signs of business quality.

None of the stocks in this comparison currently pay a material dividend.

All are effectively zero-yield and should be held for capital appreciation rather than income.

For long-horizon retirement investors, Ardelyx, Inc.

(ARDX) is the stronger choice — it scores higher on the combination of lower volatility, dividend reliability, and long-term compounding (low volatility (β 0. 87), +263. 5% 10Y return). Both have compounded well over 10 years (ARDX: +263. 5%, CSTL: -10. 1%), confirming both are viable long-term holds — but the lower-volatility option typically results in less emotional selling during corrections. Retirement portfolios generally favour predictability over maximum returns. Consult a financial advisor before making allocation decisions.

Both stocks operate in the Healthcare sector, making this a peer-level intra-sector comparison — the same macro tailwinds and headwinds will affect both.

In terms of investment character: CSTL is a small-cap quality compounder stock; ARDX is a small-cap high-growth stock. These fundamental differences mean investors should not choose between them on a single metric — the "better stock" depends entirely on which of these characteristics aligns with your investment strategy.