UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 8-K

CURRENT REPORT PURSUANT TO SECTION 13 OR 15(d) OF

THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of Earliest Event Reported): May 6, 2026

Quest Diagnostics Incorporated

(Exact Name of Registrant as Specified in Its Charter)

Delaware

(State or other jurisdiction of incorporation)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company   ¨

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.       ¨

Item 1.01 Entry into a Material Definitive Agreement

On May 6, 2026, Quest Diagnostics Incorporated (the “Company”) issued $500,000,000 aggregate principal amount of 5.000% senior notes due 2036 (the “Notes”).

The Company will pay interest on the Notes on June 30 and December 30 of each year, beginning on December 30, 2026.

The Notes will mature on June 30, 2036. The Notes will be the senior unsecured obligations of the Company and will rank equally with the Company’s other existing and future senior unsecured obligations. The Notes will not be entitled to the benefit of any sinking fund.

The Notes were issued pursuant to an indenture dated as of June 27, 2001 among the Company, the guarantors (as defined therein) and The Bank of New York Mellon, as trustee (the “Trustee”), as supplemented from time to time, and as further supplemented by a twenty-fourth supplemental indenture dated as of May 6, 2026 between the Company and the Trustee (collectively, the “Indenture”). The Indenture contains covenants that, among other things, will limit the ability of (i) the Company and certain of its subsidiaries to create certain liens and enter into certain sale and leaseback transactions and (ii) the Company to consolidate, merge or transfer all or substantially all of the Company’s assets on a consolidated basis. The Indenture provides for customary events of default. Upon a change of control triggering event (as defined in the Indenture), the Company will be required to make an offer to purchase the Notes at a price equal to 101% of their principal amount plus accrued and unpaid interest to the repurchase date.

The foregoing description of the Indenture does not purport to be complete and is qualified in its entirety by reference to the text of the applicable agreements, each of which is included as an exhibit to this Current Report on Form 8-K and incorporated by reference herein.

A copy of the opinion of Allen Overy Shearman Sterling US LLP, counsel to the Company, relating to the legality of the Notes is filed as Exhibit 5.1 to this Current Report on Form 8-K.

Item 9.01 Financial Statements and Exhibits

*        Filed herewith.

Signature

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, hereunto duly authorized.

May 8, 2026