January 8, 2024 Transforming Disease Management

2 Disclaimers This presentation contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended, which are subject to the “safe harbor” created by those sections. These forward-looking statements include, but are not limited to, statements concerning: our expected 2023 revenue of at least $210 million; our preliminary results for the fourth quarter of 2023 and the full-year 2023; our expected launch of our pipeline expansion by the end of 2025; the potential of DecisionDx-Melanoma test results to help inform more personalized patient management decisions; our belief that DecisionDx-SCC (i) could be used to guide ART the way PNI does in existing guidelines, (ii) may be a benefit to ART when appropriate high-risk patients are identified, (iii) can result in a 40% or greater reduction in risk, (iv) may inform appropriate clinical management decisions related to ART administration in high-risk SCC patients and (v) has the potential for significant cost savings to our healthcare system by helping ensure the appropriate patients receive costly therapies; our mission, vision, strategic guideposts and our strategies for driving long-term growth through strong execution and our operational guideposts; the timing and achievement of program milestones; our estimated U.S. total addressable market for our commercially available tests; our positioning for continued growth; our ongoing studies generating data and their impact on driving adoption of our tests; and study observations and interpretations of study data, including conclusions about the benefits and impact of our tests on treatment decisions and patient outcomes. The words “anticipates,” “can,” “could,” “estimates,” “expects,” “may,” “potential,” “target” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. We may not actually achieve the plans, intentions, or expectations disclosed in our forward-looking statements and you should not place undue reliance on our forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements that we make. These forward-looking statements involve risks and uncertainties that could cause our actual results to differ materially from those in the forward-looking statements, including, without limitation: our estimates and assumptions underlying our estimated U.S. total addressable market for our commercially available tests; the accuracy of our assumptions and expectations underlying preliminary fiscal 2023 results and three-year revenue and other financial targets and guidance (including, without limitation, our assumptions or expectations regarding continued reimbursement for our DecisionDx-SCC test at the current rate and reimbursement for our other products and subsequent coverage decisions, our estimated total addressable markets for our products and product candidates and the related expenses, capital requirements and potential needs for additional financing, the anticipated cost, timing and success of our product candidates, and our plans to research, develop and commercialize new tests and our ability to successfully integrate new businesses, assets, products or technologies acquired through acquisitions),the effects of macroeconomic events and conditions, including inflation and monetary supply shifts, labor shortages, liquidity concerns at, and failures of, banks and other financial institutions or other disruptions in the banking system or financing markets and recession risks, supply chain disruptions, outbreaks of contagious diseases (such as the COVID-19 pandemic) and geopolitical events (such as the ongoing Israel-Hamas War and Ukraine-Russia conflict), among others, on our business and our efforts to address its impact on our business; subsequent study or trial results and findings may contradict earlier study or trial results and findings or may not support the results discussed in this presentation, including with respect to the diagnostic and prognostic tests discussed in this presentation; actual application of our tests may not provide the anticipated benefits to patients; and the risks set forth under the heading “Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2022, and in our other filings with the SEC. The forward-looking statements are applicable only as of the date on which they are made, and we do not assume any obligation to update any forward- looking statements, except as may be required by law. Forward-Looking Statements

January 8, 2024 Preliminary Q4 2023 Information

4 Estimated ~$8B U.S. Total Addressable Market1 for Commercially Available Tests Dermatology Gastroenterology Mental Health Cutaneous melanoma/ risk of metastasis, SLNB positivity risk Patients classified as Stage I, II or III2 ~$540M ~130K Cutaneous squamous cell carcinoma/risk of metastasis Patients w/high-risk features2 ~$820M ~200K Suspicious pigmented lesions/melanoma status Patients w/ diagnostically ambiguous lesions ~$600M ~300K Barrett’s esophagus/risk of progression to esophageal cancer Patients receiving upper GI endoscopies/year who meet the intended use criteria for TissueCypher3 ~$1B ~415K Guide mental health therapy selection Based on indicated use of IDgenetix for patients diagnosed with depression, anxiety and other mental health conditions ~$5B Tests in pipeline add an additional estimated ~$5.7B to our U.S. TAM 1U.S. TAM = Total addressable market based on estimated patient population assuming average reimbursement rate among all payors.2Annual U.S. incidence for Stage I, II or III melanoma estimated at 130,000; annual U.S. incidence for squamous cell carcinoma estimated at 1,000,000 with addressable market limited to carcinomas with one or more high risk features; annual U.S. incidence for suspicious pigmented lesion biopsies estimated at 2,000,000 with addressable market limited to the 15% with an indeterminant biopsy.3415,000 upper GI endoscopies/year with confirmed dx of BE (ND, IND, LGD, EXCLUDING HGD) x $2,513 = U.S. only TAM of ~$1 billion

5 Preliminary Fourth Quarter and Year-end 2023 Performance Results1,2 (unaudited) Expect to report >$210M in revenue for 2023, exceeding the floor of our guidance of at least $200M3 1Castle expects to report its audited consolidated financial statements and complete financial and operational results for its fiscal year ended December 31, 2023, in February 2024. 2The effects of macroeconomic events and conditions continue to evolve and bring along with them a high level of uncertainty surrounding potential future effects. Therefore, trends in revenues and test report volumes are not necessarily indicative of our results of operations that can be expected for future fiscal periods. 3Castle previously provided guidance for full-year 2023 total revenue of at least $200 million. 202220234Q224Q23 $137MExpected to exceed floor of guidance3$38.3MTo be announcedRevenue 44,41970,42912,64420,284Total test reports 27,80333,3307,3018,591DecisionDx-Melanoma 5,96711,4421,8453,530DecisionDx-SCC 3,5613,9628221,018 MyPath Melanoma and DiffDx- Melanoma aggregate 2,1289,1001,0303,441TissueCypher 3,24910,9211,2143,299IDgenetix 1,7111,674432405DecisionDx-UM

6 2023 Year-end Cash, Cash Equivalents & Marketable Investment Securities Expected To Be Approximately $243 million1 Cash, Cash Equivalents & Marketable Investment Securities As of 12/31/23 ~$243M (expected) As of 9/30/23 $230M As of 6/30/23 $226M As of 3/31/23 $232M 1This amount is preliminary, unaudited and subject to adjustment as a result of, among other things, the completion of financial closing procedures, including the audit by our independent registered public accounting firm of our financial statements. As a result, amounts may differ from the amount that will be reflected in our financial statements as of and for the year-ended December 31, 2023. Accordingly, undue reliance should not be placed on this preliminary information.

Third Quarter 2023 As previously reported Transforming Disease Management

8 Consistent Execution Of Growth Initiatives Supports Long-Term Growth 2017-2023 Revenue ($M) 2017-2022 Total Test Report Volume 1 2 *2023 revenue guidance of at least $200 million, as of November 2, 2023 $14 $23 $52 $63 $94 $137 ≥$210* 2017 2018 2019 2020 2021 2022 2023E 10,614 13,445 17,055 18,185 28,145 44,419 2017 2018 2019 2020 2021 2022 53%+

9 Financial Performance Summary Q3 2023 18,409 12,390 Total test reports Total Dermatology test reports Q3 2023 Q3 2022 1See Non-GAAP reconciliations at the end of this presentation. $61.5M $60.6M Revenues Adj. Revenues1 77.9% 81.3% Gross Margin Adj. Gross Margin1 12,114 9,824 $37.0M $37.3M 69.8% 76.2% $5.0MOperating Cash Flow $229.8M Cash, Cash Equivalents & Marketable Investment Securities as of end of period $(5.2)M $266M $(6.9)M $6.6M Net Loss Adj. EBITDA1 $(20.2)M $(9.6)M

10 Raised 2023 revenue guidance to at least $200 million, from at least $180 million Earned a Top Workplaces National Industry Award, ranking third among 84 Top Workplaces in the healthcare industry Key Q3 2023 and Recent Accomplishments Publication of study data showed the addition of drug-drug interactions and lifestyle factors to drug-gene interactions provided by our IDgenetix test improved remission rates for patients with moderate to severe depression Released early discovery data regarding our inflammatory skin disease pipeline test, confirming the existence of molecular differences among inflammatory skin lesions Delivered excellent Q3 results, highlighted by $5 million in cash flow from operations and strong year-over- year growth in our total test report volume (+52%) and revenue (+66%) Multiple data announcements demonstrating the clinical utility of our TissueCypher test in guiding risk-aligned care for patients

11 Answering Clinical Questions To Guide Care Along The Patient Journey Our focus is on diagnostic support, risk stratification and therapy selection/response areas of the patient care continuum Dermatology Ophthalmology Gastroenterology Mental Health Screening Diagnostic Support Risk Stratification Therapy Selection/Response MRD/Recurrence MonitoringPATIENT CARE JOURNEY Inflammatory Skin Disease Pipeline Test

12 Significant Scientific Evidence Through Robust Clinical Research Program 237 Committed/contributing clinical research sites as of Q3 2023 ~20,460 Patients enrolled in studies over lifetime of Castle1 ~11,670 Current patients enrolled in studies as of Q3 2023 Data as of September 30, 2023 1Number reflects studies that span Castle’s dermatology, ophthalmology, gastroenterology and mental health portfolios, as well as tests that are currently in our development pipeline. Data for TissueCypher, IDgenetix and MyPath are reflective of studies that have occurred since Castle’s acquisition of the tests. 2SEER cancer registries linked CM cases diagnosed from 2013-2018 to data for patients with stage I-III CM tested with the 31-GEP as of Dec. 31, 2022; includes patients in studies not yet published 3SEER cancer registries linked UM cases diagnosed in 2018 for patients with primary uveal melanoma tested with the 15-GEP; includes patients in studies not yet published. 14 Ongoing clinical research studies Ongoing collaboration with NCI/SEER has allowed for analyses of 9,200+ patients clinically tested with DecisionDx-Melanoma2 and 2,900+ patients clinically tested with DecisionDx-UM3 to date

13 First-To-Market Dermatologic Franchise, Additional Growth Opportunities Diagnostic Support Risk Stratification Therapy Selection 1 New=clinician who ordered a Castle dermatologic test for the first time. Inflammatory Skin Disease Therapy Selection Pipeline Program Strong provider growth and continued adoption with approximately 1,600 new1 ordering clinicians and approximately 7,900 total ordering clinicians for dermatologic tests for the nine-months ended September 30, 2023

Demonstrated clinical validity, utility and impact, backed by more than 45 peer-reviewed publications, including two recent publications (Bailey et al. and Dhillon et al.) demonstrating an association with testing and improved patient outcomes Classified as an Advanced Diagnostic Laboratory Test (ADLT) by the Centers for Medicare & Medicaid Services (CMS), meaning it provides new clinical diagnostic information that cannot be obtained from any other test or combination of tests1 Reimbursement coverage through Medicare Administrative Contractor (MAC), Noridian, the MAC that oversees our Phoenix laboratory and is part of the MolDX® program that assesses molecular diagnostic technologies ADLT Status Reimbursement 1Centers for Medicare & Medicaid Services: https://www.cms.gov/Medicare/Medicare-Fee-for-Service-Payment/ClinicalLabFeeSched/Downloads/Guidance-for-Laboratories-on-ADLTs.pdf Clinical Validity, Utility and Demonstrated Patient Outcomes

15 DecisionDx- Melanoma Class Result Breslow Thickness Ulceration Mitotic Rate Age Breslow Thickness Ulceration Mitotic Rate SLN Status Age Tumor Location Individual Risk of Sentinel Lymph Node Positivity Individual Risk of Recurrence Whitman et al. JCO PO 2021; Jarell et al. JAAD 2022 Neural network with AI algorithm DecisionDx-Melanoma Provides Answers For Two Critical Clinical Questions DecisionDx-Melanoma test report DecisionDx-Melanoma test results predict a patient’s individual risk of recurrence and individual risk of sentinel lymph node positivity using two proprietary algorithms

16 DecisionDx-Melanoma Has Consistent And Independent Evidence Of Prognostic Value Across Studies Greenhaw et al. JAAD 2020 1086420 100 80 60 40 20 0 Time (Years) % R ec ur re nc e Fr ee 1086420 100 80 60 40 20 0 Time (Years) % D is ta nt M et as ta si s F re e RFS DMFS Breslow thickness (per mm) 1.12 (1.03-1.22), p=0.01 1.14 (1.02-1.26), p=0.02 Ulceration 1.63 (1.18-2.25), p=0.003 2.03 (1.48-2.78), p<0.001 Age (per year) 1.01 (0.99-1.03), p=0.60 1.00 (0.98-1.03), p=0.65 SLNB 2.42 (1.88-3.10), p<0.001 2.80 (2.07-3.77), p<0.001 31-GEP test (DecisionDx-Melanoma) 2.90 (2.01-4.19), p<0.001 2.75 (1.76-4.32), p<0.001) FEATURE HR RFS (95% CI) p-value HR DMFS (95% CI) p-value Class 1A Lowest Risk Class 1B/2A Increased Risk Class 2B Highest Risk • Quantifies expression of 31 genes from primary tumor using RT-PCR • Applies validated algorithm 31-GEP class result remains a consistent component of all DecisionDx-Melanoma reports

17 DecisionDx-Melanoma Results Guided Treatment Decisions And Led To Improved Patient Outcomes Key Findings: • Routine surveillance imaging in SLN-, high-risk patients detected melanoma recurrence ~10 months earlier than those without routine imaging. • Tumor burden at detection was significantly lower in patients tested compared to those not tested (27.6mm vs 73.1mm) • At study end, patients tested had better overall survival than those not tested (76% vs 50%, p- value= 0.027) Tested Group (n=307): “Patients who received routine imaging after high- risk GEP test scores had an earlier recurrence diagnosis with lower tumor burden, leading to better clinical outcomes.” Dhillon et al. Archives of Derm Research 2023. Independent, multi-center study of SLN negative patients (n=634) Adhered to routine imaging every 6-12 months Untested Control Group (n=327): Patients without testing Imaging driven by clinical symptoms or physical exam findings CLASS 2A/2B

Collaboration with the National Cancer Institute Linking DecisionDx-Melanoma clinical testing with patients captured in the National Cancer Institute’s SEER Program Registries

19 NCI/SEER Data Linked With DecisionDx-Melanoma Test Results Data analysis of real-world, unselected, clinically tested patients showed that DecisionDx-Melanoma testing was associated with lower melanoma-specific and overall mortality relative to untested patients Bailey et al. JCO PO, 2023. ‡Hazard ra o (HR) was computed using the untested pa ents as reference for 31-GEP tested cohort. An HR less than 1.0 demonstrates improved survival in 31-GEP tested patients. Diagnosis and 31-GEP test date 2016-2018. AJCC8=American Joint Committee on Cancer Eighth Edition Data show DecisionDx-Melanoma provided significant, independent risk stratification of patients with cutaneous melanoma, beyond AJCC8 stage, which may help inform more personalized patient management decisions Tested: n=3,258 Not Tested: n=9,774 29% 17% Lower 3-year melanoma-specific mortality rate in patients clinically tested vs untested Lower 3-year overall mortality rate in patients clinically tested vs untested Deaths, % (n/N)3-year MSS (95% Cl)Group 1.7% (57/3258)97.4% (96.6-98.2%)31-GEP Tested 2.5% (242/9774)96.1% (95.5-96.6%)Matched Untested 0.71 (0.53-0.94) p=0.018Hazard Ratio Deaths, % (n/N)3-year OS (95% Cl)Group 5.2% (170/3258)92.4% (91.1-93.6%)31-GEP Tested 6.2% (610/9774)90.9% (90.1-91.7%)Matched Untested 0.83 (0.70-0.99) p=0.034Hazard Ratio ‡ ‡