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Castle Biosciences

CSTL on Nasdaq. Castle Biosciences sells cancer diagnostic tests to clinicians. Market value $1.1bn.

Watch this stockFree. We tell you when something changes.

Price checks use the past 12 months to June 2026. Quality checks use five annual reports, the latest for the year to December 2025.

Should I look at this?

Not a fit for our list right now

Why it could be worth it

What to watch out for

See Health care stocks that passed both tests

This is not advice. Check the numbers below.

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Cash yield
past 12 months to June 2026
-0.0%low

For every $100 of what the whole company costs, it produced $-0.04 of spare cash in the past 12 months. A savings account pays about $4.

Price to profit
past 12 months to June 2026
n/a

The filings do not give us enough to work this out.

Return on capital
five annual reports to December 2025
-9.3%five-year median

Each dollar kept in the business earns -9 cents a year. Above 10 is good.

Quality score: 48 of 100. Price score: 0 of 100. Our list needs 70 on quality and 60 on price.

$36.65 a share, 107% above its 1-year low

Over the past year the price has ranged from $17.72 to $44.28.

Pays no dividend

Prices from Monday’s close (5 October).

Five years of cash, in billions

-0.0
-0.0
-0.0
0.0
0.0
-0.0
2021202220232024202512 monthsto Jun '26

Spare cash swings from quarter to quarter here: a shortfall of $489,000 in the past 12 months, $28 million in the year to December 2025.

Revenue
$94m$137m$220m$332m$344m
Operating margin
-42.6%-53.2%-30.9%2.6%-12.4%
Debt to equity
n/an/a0.000.020.02
Shares outstanding
0.03bn0.03bn0.03bn0.03bn0.03bn

Health checks

  • Free cash flow positive2 of 5 years
  • Accounting looks honest (Beneish)Nothing unusual
  • Financial strength (Piotroski)2 of 9
  • Profit backed by cash (accruals)No
  • Debt0.02× equity
  • Revenue growth, five yearsStrong, 40.6% a year
  • Buying back its own sharesNo, 16% more shares since 2021

The quarter to June 2026

How the business did, compared with the same quarter a year earlier.

  • Sales: $104 million last quarter, up 20% on a year ago.
  • A loss of $2 million, after a profit of $5 million a year ago.
  • It loses 9 cents on each $1 of sales, compared with 7 cents a year earlier.
  • Over the past 12 months it spent $489,000 more cash than it brought in. A year earlier it had $35 million spare.
  • 3% more shares than a year ago. Each share owns a bit less of the company.
  • It has $52 million more cash than debt, down from $72 million a year ago.
  • Sales grew on a year ago in 2 of the last 4 quarters.
Sales by quarter
Sales by quarter
Quarter toAmount
September 2024$86m
December 2024$86m
March 2025$88m
June 2025$86m
September 2025$83m
December 2025$87m
March 2026$84m
June 2026$104m
Profit by quarter
Profit by quarter
Quarter toAmount
September 2024$2m
December 2024$10m
March 2025-$26m
June 2025$5m
September 2025-$501,000
December 2025-$2m
March 2026-$15m
June 2026-$2m

From the company's quarterly reports to the SEC.

Dates

Next results (estimated)
n/a
Last annual report (10-K)
26 February 2026
Next quarterly (estimated, 10-Q)
29 October 2026

Who owns it

None of the long-term investors we follow own it. 176 funds in all.

Jun '25
Dec '25
Jun '26
Largest holders overall

From 13F filings for the quarter ended 30 June 2026.

Big holders and activists

3 investors own more than 5%.

From Schedule 13D and 13G filings: anyone owning more than 5% must file one.

What insiders did

No insider bought shares on the open market in the last 12 months. 5 sold $18m, $15m of it under preset trading plans.

  • Juvenal Tobin W
    Chief Commercial Officer
    Sold
    under a preset trading plan
    Date
    30 September 2026
    Shares
    1,034
    Price
    $34.20
    Value
    $35,363
  • Juvenal Tobin W
    Chief Commercial Officer
    Sold
    under a preset trading plan
    Date
    29 September 2026
    Shares
    5,543
    Price
    $35.50
    Value
    $196,765
  • MAETZOLD DEREK J
    Pres. & Chief Exec. Officer, Director
    Sold
    under a preset trading plan
    Date
    21 September 2026
    Shares
    14,496
    Price
    $35.33
    Value
    $512,202
  • Juvenal Tobin W
    Chief Commercial Officer
    Sold
    under a preset trading plan
    Date
    15 September 2026
    Shares
    3,294
    Price
    $35.14
    Value
    $115,758
  • Juvenal Tobin W
    Chief Commercial Officer
    Sold
    under a preset trading plan
    Date
    14 September 2026
    Shares
    3,306
    Price
    $32.32
    Value
    $106,863
  • MAETZOLD DEREK J
    Pres. & Chief Exec. Officer, Director
    Sold
    under a preset trading plan
    Date
    11 September 2026
    Shares
    14,518
    Price
    $32.02
    Value
    $464,870
  • Oelschlager Kristen M
    Chief Operating Officer
    Sold
    under a preset trading plan
    Date
    1 September 2026
    Shares
    4,152
    Price
    $33.39
    Value
    $138,643
  • Juvenal Tobin W
    Chief Commercial Officer
    Sold
    under a preset trading plan
    Date
    31 August 2026
    Shares
    1,730
    Price
    $32.81
    Value
    $56,761
  • MAETZOLD DEREK J
    Pres. & Chief Exec. Officer, Director
    Sold
    under a preset trading plan
    Date
    28 August 2026
    Shares
    18,560
    Price
    $33.20
    Value
    $616,134
  • Juvenal Tobin W
    Chief Commercial Officer
    Sold
    under a preset trading plan
    Date
    25 August 2026
    Shares
    9,050
    Price
    $35.04
    Value
    $317,112

From Form 4 filings: insiders must report trades in their own company's shares within two days.

Warning signs in its filings

Problems the company itself reported to the SEC, in its own words.

None of the warning signs we check for were found.

We look for five warning signs: doubt it can keep going, weak checks on its own accounts, a notice that its past accounts can't be relied on, a change of auditor, and one customer bringing in a big share of sales. We don't check lawsuits, investigations or debt yet.

We checked the auditor's report, internal controls, restatement notices, auditor changes and big customers in the 10-K filed 26 Feb 2026, plus the 10-Q filed 30 Jul 2026 and 3 later 8-Ks.

A 10-K is the yearly report every US company files with the SEC. An 8-K is a short notice of a big event.

What could go wrong

Cheap for a reason is the question the numbers cannot answer.

Whether the price already reflects the risks is what the deep dive is for.

The deep dive

Everything above is arithmetic on public filings. The deep dive reads the last ten years of annual reports, the proxy statements, and the earnings calls, then argues the case the way Buffett, Klarman, and Hohn would, and checks every claim against the source.

  • What the business is worth, as a range, and the margin of safety at today’s price
  • Prices to start buying, buy, and buy hard
  • The three things that would make this a mistake
  • Every number footnoted to the filing it came from
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What a finished deep dive looks like

Not advice. Numbers on this page come from SEC filings and are updated each night; prices are updated again after the US market closes. The five-year figures are rounded.