How Claims Work

Why advocates need the big picture

A medical bill is the end of a long chain. Care happens first; then documentation, coding, claim submission, insurance review, and payment processing—all before the patient sees a final balance. When something goes wrong, advocates who know the sequence can ask the right office whether the problem is missing documentation, a coding choice, a billing typo, or an insurer denial—instead of treating every bill as a single mystery charge.

This guide walks through that chain in plain language. It is part of Medical coding 101, after Why Coding Matters. You do not need to memorize claim form fields; you need a mental map of who touches the claim and when patient-facing documents appear.

The claim flow, step by step

1. The encounter

The patient receives care: office visit, lab draw, imaging, surgery, ER treatment, infusion, or hospital stay. At check-in, staff collect insurance information (and sometimes copay). The encounter itself does not create a claim—it creates the clinical facts and often a charge capture record (what services were ordered or performed) that downstream teams rely on.

2. Documentation

Clinicians and staff document what happened in the medical record: chief complaint, exam, diagnoses, procedures, medications, medical decision-making, and discharge instructions. Documentation must support any codes billed later. If the chart is incomplete, delayed, or does not support the level of service billed, payers may deny payment or auditors may flag the claim. Advocates rarely see the chart first, but they can request records when codes on the bill do not match the patient's account of care.

3. Coding

Coding staff—or the physician with EHR tools—assign ICD-10-CM diagnosis codes and CPT/HCPCS procedure codes based on the documentation. Modifiers, units, place of service, and dates of service are added where rules require them. This step translates care into the standardized language payers use. See Medical Coding vs. Billing and the upcoming Key Code Sets guide for what those fields mean on a statement.

4. Claim build

Billing staff assemble the claim: patient demographics, subscriber and group numbers, referring provider if needed, prior authorization numbers, diagnosis and procedure codes, charges, and place of service. For professional services this is often a CMS-1500 form electronically; hospitals use UB-04 (institutional) claims with revenue codes and different fields. Errors here—wrong member ID, missing authorization, transposed date of birth—can cause rejection before the insurer even adjudicates medical coverage.

5. Submission to payer

The claim is sent electronically (typically) to the primary insurer, Medicare, Medicaid, or other payer. Clearinghouses may sit in the middle. The provider may batch claims daily or weekly. Until submission happens, insurance has nothing to process—the patient may still receive a statement from the provider asking for payment, but there is no EOB yet. If weeks pass with no EOB, ask whether a claim was filed and to which payer.

6. Insurance review

The payer adjudicatesthe claim: checks eligibility, network status, benefits, prior authorization, medical policy, and coding edits. The outcome is paid (full or partial), denied, or pended for more information. Edits may downcode, bundle lines, or reject invalid code pairs automatically. Medical necessity denials happen here—not in the provider's billing office. Use the Denial Decoder when the EOB shows a denial reason.

7. Payment and EOB

When the payer decides, it sends payment to the provider (or patient in rare cases) and produces a remittance advice explaining what was allowed, paid, adjusted, and assigned to patient responsibility. Patients usually see this as an Explanation of Benefits (EOB)—not a bill. The EOB shows billed charges, allowed amounts, adjustments, insurer payment, and your share (copay, deductible, coinsurance). See Medical Bill vs. EOB and Coding, Charges & Allowed Amounts.

8. Patient statement

The provider billing office posts insurance payment and contractual adjustments, then bills the patient for remaining balance per the EOB (plus any copay collected at visit that was not yet applied). Timing varies: some bills arrive before the EOB, some long after. The patient statement should reflect adjudication results for in-network care; when it does not, that is a billing reconciliation problem. If the patient never pays, the account may go to collections—see Collections, Credit & Medical Debt.

Where mistakes usually happen

Advocates can map many problems to a stage in the flow:

• Documentation: chart does not support billed visit level or procedure—fix may require physician addendum or corrected coding, not just a billing call
• Coding: wrong code, duplicate code, wrong diagnosis link—often fixed with a corrected claim and new EOB
• Claim build / submission: wrong insurance, missing auth, never submitted—billing fixes and resubmits
• Adjudication: denial for medical necessity, network, or benefit— insurance appeal path; provider may hold bill
• Patient billing: insurance payment not posted, balance exceeds EOB—provider billing dispute; see Billing Dispute Roadmap

Mistakes early in the chain (coding, submission) often look like insurance denials or wrong EOBs. Mistakes late in the chain look like bills that do not match the EOB the patient already received.

Scenarios advocates run into

Bill arrives before the EOB

The provider mailed a statement while the claim is still pending or before payment posted. The patient thinks they owe the full billed charge. Check whether a claim was submitted and wait for the EOB—or call the insurer to confirm receipt. Payment decisions belong to the adjudication step; the bill should eventually reflect them.

Insurance says no claim on file

The patient has coverage but the insurer has no record of the service. Possible causes: provider never billed, claim went to wrong payer, demographic mismatch caused rejection, or timing (claim very recent). Ask the provider billing office for submission date and payer; ask insurance for rejection reason if rejected. The fix is usually resubmission—not patient self-pay at chargemaster rates.

Denied claim, full bill sent

Adjudication denied the claim; billing still sent a bill for the full charge. The patient may owe nothing until appeal is resolved, or only cost-sharing if partial payment applied. Identify whether to appeal the insurer, request a corrected claim from the provider, or both. Do not pay the full list price while a coverage dispute is open without understanding surprise billing or hold policies.

One visit, many claims

After ER care, the patient receives three EOBs and five bills from different entities. Each claim went through its own submission and adjudication. Organize by date of service and provider; match each bill to its EOB separately. What looks like duplicate billing may be legitimate separate professional and facility claims.

Corrected claim and new EOB

Billing or coding fixes an error and submits a corrected claim (often replacing the original). A new EOB arrives with different allowed amounts or patient responsibility. Wait for the updated EOB before paying; ask billing to reissue the patient statement after the second adjudication. Old bills may still show in portals until the account is updated.

Primary paid, secondary pending

Patients with two plans may see primary EOB first with remaining balance, then secondary processes a coordination-of-benefits claim. Patient responsibility can change after secondary pays. The flow adds another adjudication round—bills sent after primary only may overstate what is owed. Ask whether secondary billing was triggered.