ICD-10 code A78: Q fever diagnosis and billing guide

Key Takeaways

ICD-10 code A78 is the billable diagnosis code for Q fever, a zoonotic bacterial infection caused by Coxiella burnetii.

A78 is a single terminal code covering both acute and chronic Q fever; ICD-10-CM provides no further subcategories.

Never substitute spotted fever codes (A77) or other rickettsioses (A79) for A78 when Q fever is confirmed.

Pabau’s claims management software supports accurate ICD-10 diagnosis attachment, reducing coding errors on infectious disease claims.

ICD-10 code A78 is the sole billable code for Q fever (Coxiella burnetii infection) in ICD-10-CM. This guide covers the definition, classification, clinical criteria, documentation requirements, related codes, and billing context for A78.

Q fever is a notifiable condition in the US and most high-income countries, requiring documentation that satisfies both billing and public health reporting obligations.

ICD-10 code A78: Definition and clinical overview

ICD-10 code A78 classifies Q fever, a bacterial infection caused by the obligate intracellular organism Coxiella burnetii. The code sits within Chapter 1 (Certain infectious and parasitic diseases, A00-B99) under the Rickettsioses block (A75-A79), as maintained by the WHO ICD-10 browser.

Coxiella burnetii spreads primarily through inhalation of contaminated aerosols from infected animals, most commonly cattle, sheep, and goats. Parturient animals shed the organism in high concentrations through birth products, urine, feces, and milk. Farmers, veterinarians, abattoir workers, and laboratory personnel face elevated occupational exposure risk.

The CDC/NCHS ICD-10-CM web tool confirms A78 as a fully billable, specific diagnosis code. It carries no Type 1 or Type 2 Excludes notes and is valid for submission on outpatient and inpatient claims for the 2026 fiscal year. Coders working with claims management software should verify the code against the annual CMS update files each October.

A78 code classification and synonyms

The “Applicable To” notes in the CMS ICD-10-CM tabular list identify three clinical synonyms that map to A78: Infection due to Coxiella burnetii, Nine Mile fever (named after Nine Mile Creek in Montana, where the organism was first isolated), and Quadrilateral fever. When any of these terms appears in the medical record, A78 is the correct code.

Documentation referencing “Q fever hepatitis,” “Q fever endocarditis,” or “Q fever pneumonia” should be evaluated for additional codes before filing, but A78 remains the primary diagnosis code for Q fever regardless of the affected organ system.

Accurate synonym recognition reduces the risk of coders defaulting to a less specific rickettsioses code. See the Chlamydia psittaci infections (psittacosis) ICD-10 coding guide for how synonym mapping works across other respiratory zoonotic codes; the principle is identical here.

Clinical presentation and diagnosis criteria for A78

Clinicians working in primary care, infectious disease, and occupational health will encounter Q fever most often in patients with recent animal or farm exposure. The functional and integrative medicine settings that see patients with complex fatigue presentations may also encounter chronic Q fever.

Acute Q fever typically presents within two to three weeks of Coxiella burnetii exposure. Common features include:

• High fever, often 39-40°C, lasting one to two weeks
• Severe headache, frequently retro-orbital
• Myalgia and fatigue
• Non-productive cough with atypical pneumonia on imaging in roughly 50% of cases
• Abnormal liver function tests consistent with granulomatous hepatitis
• Rash, present in fewer than 20% of cases

When imaging shows lobar consolidation rather than a diffuse atypical pattern, rule out co-existing bacterial pneumonia, such as pneumonia due to Streptococcus pneumoniae (see the ICD-10 Code J13 guide), before assigning an additional organ-specific code alongside A78.

Chronic Q fever, defined as infection persisting beyond six months, occurs in a minority of patients. Endocarditis is the most serious chronic presentation, accounting for the majority of Q fever fatalities. Pre-existing valvular disease, pregnancy, and immunosuppression are recognized risk factors for progression to chronic disease.

Diagnosis is confirmed through serological testing, primarily phase I and phase II immunofluorescence assay (IFA) titers. The CDC recommends a phase II IgG titer of at least 1:128 for acute disease confirmation.

A78 is assigned on the basis of confirmed diagnosis; do not assign it for suspected or possible Q fever pending laboratory results. Use the appropriate signs and symptoms code instead until confirmation.

Consistent documentation of the clinical basis for diagnosis supports clean claims and enables accurate patient record-keeping across follow-up encounters.

Documentation requirements for A78

A78 is a single-code assignment for confirmed Q fever. The supporting documentation must meet the following standards to support billing and public health reporting obligations.

A complete encounter note for A78 should include:

• Confirmed diagnosis statement: The note must explicitly state “Q fever” or a recognized synonym. Avoid vague terms such as “rickettsiosis, NOS” when the specific organism is known.
• Laboratory evidence: Document the serological test used, the result, and the date. Phase II IgG titers are standard; include the reference lab’s interpretation.
• Exposure history: Record the suspected exposure source (occupational animal contact, geographic travel to an endemic area) and the estimated exposure date.
• Affected organ systems: If Q fever involves the liver (granulomatous hepatitis), lungs (atypical pneumonia), or heart (endocarditis), document each system. Additional codes may apply alongside A78.
• Acute vs chronic distinction: While ICD-10-CM does not offer a separate chronic Q fever subcode, the clinical note should specify the disease phase because it affects treatment planning, follow-up intervals, and public health notification.

Q fever is a nationally notifiable disease in the US under CDC surveillance reporting requirements. Clinicians should verify state-specific notification timelines; these obligations exist independently of the billing code but the same encounter documentation supports both. Using digital intake and clinical forms with structured fields for exposure history and laboratory results reduces incomplete documentation at the time of filing.

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Related ICD-10 codes and differential coding for Q fever

Coders frequently need to distinguish A78 from adjacent codes within the Rickettsioses block and from other infectious disease codes that share clinical features. The table below covers the most common differential coding decisions.

The key differentiator for A78 is laboratory-confirmed Coxiella burnetii infection. When serology confirms Q fever specifically, A79.9 (Rickettsiosis, unspecified) is not appropriate even if the physician’s note uses generic rickettsiosis language. Query the physician if the diagnosis term does not align with the serological result.

See the sequelae of poliomyelitis ICD-10 coding guide for how specificity rules apply in other infectious disease categories, and the heart-lung transplant rejection ICD-10 reference for how terminal codes work across different ICD-10-CM chapters.

For additional crosswalk support, the AAPC Codify ICD-10-CM lookup and ICD List both provide synonym listings and related code hierarchies for A78 and the surrounding rickettsioses block.

Billing and reimbursement context for ICD-10 code A78

A78 is a low-volume code in most outpatient practices. Payers do not commonly apply frequency edits or prior authorization requirements specifically to Q fever, but the infectious disease category as a whole receives heightened scrutiny from Medicare and commercial payers when documentation is sparse.

Common billing scenarios for A78 include:

• Outpatient evaluation and management (E/M): Pair A78 with the appropriate E/M CPT code (99202-99215) based on complexity. Acute Q fever typically warrants a moderate-complexity or high-complexity visit given the diagnostic workup required. For lower-acuity emergency department evaluations, see the CPT Code 99283 guide.
• Infectious disease consultation: Use A78 as the primary diagnosis on the consultation claim. Document the specific reason for referral and the clinical findings that prompted the specialist request.
• Inpatient admission: For hospitalized patients with severe Q fever pneumonia or endocarditis, A78 may serve as the principal diagnosis when it drove the admission decision. Assign secondary codes for the organ complications.
• Follow-up encounters: For chronic Q fever monitoring, continue using A78 as the primary code. Document the disease phase and any change in serological status at each encounter.

Serological testing CPT codes (such as 86638 for Coxiella burnetii antibody testing) typically accompany A78 claims and should be ordered with the diagnosis code pre-populated to avoid medical necessity denials. Practices using compliance management software can set up rule-based alerts that flag missing diagnosis attachments before claims are submitted.

For practice management teams handling infectious disease coding workflows, the HIPAA compliance guide for medical offices covers the documentation retention requirements that apply to notifiable disease records.

The medical forms guide for healthcare practices outlines how structured clinical forms capture complete documentation at the point of care, and practices standardizing note-taking across an EHR may find our guide to clinical documentation software useful for evaluating structured documentation tools.

The CMS ICD-10 codes page publishes updated tabular lists and official coding guidelines each October. Verify A78 remains current and review any addenda that affect the rickettsioses block before the start of each new fiscal year.

Related recent coding references include the familial erythrocytosis billing and coding guide and the malignant neoplasm of appendix ICD-10 coding reference, both useful when coding infectious or systemic conditions alongside hematologic or oncologic diagnoses.

Summary

ICD-10 code A78 is the only code for confirmed Q fever in ICD-10-CM, covering both acute and chronic presentations without subcategory distinction. Accurate documentation of the exposure history, serological confirmation, and any organ complications supports clean claims and reduces denial and audit risk.

Pabau’s claims management tools and structured digital forms give infectious disease and primary care practices the documentation infrastructure to support ICD-10 code A78 claims accurately. Practices new to infectious disease billing can start with our medical billing overview, and a review of systems template helps standardize exposure history capture at intake.

For teams handling E/M coding alongside infectious disease diagnoses, the subsequent hospital inpatient care CPT code 99232 guide and the principal care management CPT code 99424 billing guide are useful companion references. See how it works by visiting pabau.com/book-demo.

Continue your research

Coding another respiratory zoonotic infection? The ICD-10 code A70 (psittacosis) guide walks through the synonym mapping and specificity rules that mirror the approach used for A78.

Need to tighten documentation on notifiable disease records? HIPAA compliance for medical offices covers the retention and security standards that apply to infectious disease reporting.

Want to cut coding errors on infectious disease claims? Pabau’s claims management software flags missing diagnosis attachments before submission, reducing denials on low-volume codes like A78.

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