A physician sees a patient for persistent fatigue and orders a ferritin test. The patient goes to a hospital’s outpatient laboratory for the test and receives no other hospital services that day. The result comes back low, and the hospital submits its claim. The insurer denies payment, saying the test was not medically necessary.

The hospital now has a reimbursement dispute. The physician believed the test was appropriate, and the result identified a potential medical problem.

Who gets to decide what was medically necessary, and under what standard?

Medical necessity starts with the physician’s judgment. Whether that judgment is paid for is decided by a written standard: federal rules for Medicare and Medicare Advantage, a state statute for Medi-Cal, and, for most commercial claims in California, the payer’s own documents, applied by the payer. The same test, ordered for the same reason, can produce different payment decisions.

Traditional Medicare: The Diagnosis Code Matters

Medicare does not pay for services that are not “reasonable and necessary for the diagnosis or treatment of illness or injury” (42 U.S.C. § 1395y(a)(1)(A)). For iron testing, Medicare’s National Coverage Determination 190.18 lists clinical circumstances that may support testing, among them abnormal blood counts, gastrointestinal bleeding, heavy menstrual bleeding and malabsorption. Fatigue is not on that list, but the NCD calls the listed presentations “examples,” and it does not exclude fatigue by name.

A claim may be denied when its diagnosis code does not support coverage. CMS’s denial guidance for the NCD, which CMS describes as its own interpretation, says such a claim is denied “unless other medical documentation justifying the necessity is submitted with the claim.” If the hospital submits only the diagnosis code for fatigue, the claim may be stopped by a coverage edit.

Suppose the physician’s records also document heavy menstrual bleeding or abnormal blood counts as reasons for the test. Those findings could support medical necessity under Medicare’s own criteria. The hospital can request them from the ordering physician (42 C.F.R. § 410.32(d)(3)(iii)), and any corrected claim must reflect what the physician documented. The hospital cannot add a diagnosis to obtain payment. A low ferritin result may help explain the patient’s symptoms, but it does not show that the test met Medicare’s coverage requirements when it was ordered.

The example assumes a laboratory-only visit. At hospitals paid under Medicare’s outpatient prospective payment system, outpatient lab tests are generally packaged with other hospital services and paid separately only when the patient receives no other hospital outpatient services that day (Medicare Claims Processing Manual, Ch. 16, § 30.3). A lab test included in another service’s payment has not been denied for lack of medical necessity.

Medicare Advantage: Medicare Rules Still Apply

Medicare Advantage plans must comply with applicable national coverage determinations (42 C.F.R. § 422.101). Where Medicare’s coverage criteria are not fully established, plans may develop their own internal criteria, which must be publicly accessible and “based on current evidence in widely used treatment guidelines or clinical literature.” Individual decisions must also account for the patient’s medical history, physician recommendations and clinical notes. A hospital challenging a Medicare Advantage denial should ask whether the plan applied the controlling Medicare rule and whether it was permitted to use criteria of its own.

Medi-Cal: A Different Statutory Standard

California’s Welfare and Institutions Code § 14059.5 defines a medically necessary service for adults as one that is “reasonable and necessary to protect life, to prevent significant illness or significant disability, or to alleviate severe pain.” For patients under 21, federal Medicaid law sets a broader standard, covering services to “correct or ameliorate” conditions discovered through screening. Medicare’s iron-testing criteria do not decide whether Medi-Cal must pay the same claim.

Commercial Insurance: Whose Definition Controls?

For California commercial coverage, the standard may come from the coverage document, the provider agreement and the payer’s medical policies, subject to state law. In Missionary Guadalupanas of the Holy Spirit, Inc. v. Rouillard (2019), the California Court of Appeal addressed medical necessity under the Knox-Keene Act. The court said the term cannot be limited to Medi-Cal’s statutory definition and wrote that “in broad terms a ‘medically necessary’ procedure must include a procedure that is a generally recognized treatment for a medical condition presented by the patient.” The court declined to define the term fully.

Blue Shield of California’s July 2026 Hospital and Facility Guidelines are one payer’s example. The manual covers services “established as safe and effective” and furnished under “generally accepted professional standards,” and requires that they be consistent, “as determined by Blue Shield,” with “Blue Shield Medical Policy and/or evidenced based clinical guidelines” and with “the symptoms or diagnosis.” For outpatient reimbursement, it directs hospitals to their contractual agreements. A payer’s published policy explains how it decides coverage, and the hospital’s right to payment also depends on its contract and on the law.

When a Payer’s Own Policy Creates a Dispute

Anthem’s Serum Iron Testing guideline, CG-LAB-21, last reviewed in February 2026, recognizes testing for suspected iron deficiency when associated symptoms or clinical findings are present. Its discussion acknowledges that severe iron deficiency anemia may cause fatigue. Yet its diagnosis-code list does not include R53.83, the code for other fatigue, and its coding section says the tests are not medically necessary for diagnoses not listed. A physician may therefore document fatigue and suspect iron deficiency while the hospital’s claim carries a code absent from the payer’s list.

That discrepancy is worth raising in a reimbursement appeal. It does not by itself establish that the payer owes payment: Anthem states that individual plans may choose whether to adopt the guideline and that governing law, contract language and medical policy take precedence. The hospital must establish which policy governs the patient’s coverage and whether the clinical documentation supports the test.

What New Medical Evidence Can Establish

In September 2026, the American Society of Hematology published new guidance on diagnosing iron deficiency. Its visual summary gives a conditional recommendation for a ferritin threshold of 30 ng/mL or lower for adults who are not menstruating or pregnant. An accompanying remark says a threshold of 50 ng/mL or lower is appropriate for people “with signs or symptoms associated with iron deficiency or ongoing iron deficiency risk factors.”

These recommendations help clinicians interpret a result. None of them addresses who should be tested, so they do not show that this test should have been ordered. The hospital must still show why the original test met the coverage requirements.

The guidance matters legally in two places. Where Medicare’s criteria are not fully established, a Medicare Advantage plan’s own criteria must rest on “current evidence,” and California’s independent medical reviewers weigh “Nationally recognized professional standards” (Health & Safety Code § 1374.33(b)). A newer clinical guideline does not override a binding Medicare coverage determination.

What a Hospital Should Do After a Denial

1. Identify the governing standard. Determine the payer, coverage arrangement, applicable law, medical policy and provider contract. Distinguish a contractual payment claim from an appeal pursued on the patient’s behalf.
2. Obtain the denial criteria. Identify the policy, diagnosis-code edit or clinical requirement the payer relied on. For California-regulated plans, Health and Safety Code § 1367.01 requires the denial to state “the criteria or guidelines used, and the clinical reasons for the decisions regarding medical necessity.”
3. Review the physician’s documentation. For Medicare outpatient laboratory services, 42 C.F.R. § 410.32 requires the ordering practitioner to maintain documentation of medical necessity. The billing hospital must keep the information it receives and documentation that its claim accurately reflects that information. It may request additional relevant records from the ordering physician.
4. Challenge the actual basis for denial. If the payer’s written clinical indications support testing but its code list excludes the documented symptom, explain the discrepancy. Support the appeal with the patient’s records and applicable clinical evidence.
5. Use the correct dispute process. For claims within the California Department of Managed Health Care’s jurisdiction, hospitals generally must first use the payer’s provider dispute resolution process before seeking DMHC provider-complaint review. DMHC’s instructions specify 45 working days or an earlier written determination. Since July 1, 2026, new provider complaints must concern a claim whose last date of service is within 30 months.

Patient benefit appeals, including independent medical review and ERISA claims procedures, involve separate rights and eligibility requirements. A hospital cannot assume those rights arise from its provider contract.

The Question That Determines Payment

Medical necessity is both a clinical judgment and a legal coverage standard. A physician may have sound reasons for ordering a test, and a hospital may furnish it properly and bill accurately, yet the payer may still deny the claim under its coverage criteria. The hospital must then identify the controlling standard and show what the medical record supports. The dispute is decided by asking whether the denial is justified under the law, the governing coverage documents and the evidence about this patient.