ICD-10 Code B64: Unspecified protozoal disease (FY2026)

Key Takeaways

ICD-10 Code B64 is a valid, billable diagnosis code for unspecified protozoal disease, covering cases where the causative organism cannot be identified.

B64 is valid for fiscal year 2026 (October 1, 2025 through September 30, 2026) and is accepted in all HIPAA-covered transactions.

Always use a more specific protozoal disease code (B50-B63) when the causative organism is known – B64 is the correct fallback only when no specific organism can be documented.

Pabau’s claims management software supports accurate ICD-10-CM diagnosis recording and HIPAA-compliant billing workflows for codes including B64.

ICD-10 Code B64: Definition, billable status, and FY2026 validity

ICD-10 Code B64 is the billable ICD-10-CM code for unspecified protozoal disease. It applies when a protozoal infection is confirmed but the causative organism has not been identified or documented. Most protozoal infections resolve to a known organism instead, so B64 is a fallback, not a default.

B64 is included in the 2026 edition of ICD-10-CM and can be used in all HIPAA-covered transactions from October 1, 2025 through September 30, 2026. According to the CDC/NCHS ICD-10-CM web tool, B64 sits at the terminus of the B50-B64 protozoal diseases block within Chapter 1 (Certain infectious and parasitic diseases, A00-B99). It carries no child codes, meaning it is a terminal billable code, not a category header.

Clinicians and coders searching for guidance on HIPAA-compliant diagnosis documentation should note that B64 applies only when the responsible protozoan organism has not been identified or documented. When the organism is known, a more specific code from B50 through B63 must be used instead. This reference covers every element coders need: hierarchy, synonyms, clinical application, MS-DRG mapping, and a decision table for selecting B64 versus its more specific alternatives.

B64 code at a glance

The table below consolidates the key reference data for ICD-10 Code B64 in a single view. Use this for quick verification before submitting claims or documenting diagnoses.

Where B64 fits in the ICD-10-CM hierarchy

Understanding the protozoal disease ICD-10 hierarchy helps coders verify they are using B64 correctly, not skipping over a more specific code that should apply. B64 is the final code in its block, sitting at the bottom of this structure:

• Chapter 1: Certain infectious and parasitic diseases (A00-B99)
• Block B50-B64: Protozoal diseases
• B50-B54: Malaria (Plasmodium species)
• B55: Leishmaniasis (Leishmania species)
• B56-B57: Trypanosomiasis (African sleeping sickness, Chagas disease)
• B58: Toxoplasmosis (Toxoplasma gondii)
• B59: Pneumocystosis (Pneumocystis jiroveci)
• B60: Other protozoal diseases, not elsewhere classified (includes Babesiosis, Acanthamoebiasis, Naegleria infections)
• B64: Unspecified protozoal disease (NOS – no organism identified)

Note that B61, B62, and B63 do not exist in the ICD-10-CM tabular list – B60 jumps directly to B64. Coders should also note that A07.2 (Cryptosporidiosis) falls outside the B50-B64 block, under intestinal protozoan diseases in the A00-A09 range. This matters when a patient presents with cryptosporidial infection: the correct code is A07.2, not a code from the B50-B64 block. Amebiasis – the intestinal infection caused by Entamoeba histolytica – is likewise classified under A06, outside the B50-B64 protozoal diseases block, so it is never coded to B64. Coders documenting multi-diagnosis claims should apply the same block-boundary logic to unrelated chapters, such as M15.9, to avoid sequencing errors.

The WHO ICD-10 browser confirms B64’s position in the international classification, where it carries the same description: unspecified protozoal disease. The ICD-10-CM version (used in the United States) mirrors this structure with clinical modifications for U.S. payer requirements.

Approximate synonyms for B64

The ICD-10-CM alphabetical index recognizes several alternative terms that route coders to B64. Knowing these synonyms helps during chart review when clinical documentation uses non-standard language.

• Protozoal disease, unspecified
• Protozoal infection NOS (Not Otherwise Specified)
• Protozoal disease NOS
• Unspecified protozoal infection
• Protozoosis, unspecified

The designation “NOS” is key. In ICD-10-CM coding, NOS indicates that the documentation does not provide enough information to assign a more specific code. For B64, NOS means the clinical record does not identify which protozoan organism caused the infection. This is clinically distinct from “not yet confirmed” – if testing is pending, coders should use symptoms or signs codes rather than B64, and revisit once the organism is confirmed.

Coders working with practices that handle high volumes of infectious disease documentation may find structured clinical documentation tools helpful for capturing the organism detail needed to avoid reaching for unspecified codes unnecessarily.

Clinical information: What conditions map to B64?

Protozoa are single-celled organisms that can infect humans through various routes: insect bites, contaminated water, direct contact, and food consumption. Many of these infections surface after international travel, which is why practices running travel clinic software regularly document protozoal disease. Most pathogenic protozoa have specific ICD-10-CM codes. B64 applies only in the residual scenario: protozoal infection is clinically suspected or confirmed, but the specific organism is not documented and cannot be identified from the available record.

Common protozoa that have their own specific codes include:

• Plasmodium species (malaria): B50 (Plasmodium falciparum), B51 (P. vivax), B52 (P. malariae), B53 (other specified malaria), B54 (unspecified malaria)
• Leishmania species: B55.0 (visceral leishmaniasis), B55.1 (cutaneous leishmaniasis), B55.2 (mucocutaneous leishmaniasis)
• Trypanosoma brucei / T. cruzi: B56 (African trypanosomiasis), B57 (Chagas disease)
• Toxoplasma gondii: B58 (toxoplasmosis, with sub-codes by organ involvement)
• Pneumocystis jiroveci: B59 (pneumocystosis)
• Babesia species, Acanthamoeba, Naegleria fowleri: B60 sub-codes

B64 is appropriate when the clinical record documents a protozoal infection without identifying any of the above organisms – for example, a returning traveler with a confirmed protozoal illness where species identification was unsuccessful or not pursued, or a patient with clinical and microscopic evidence of protozoal infection where organism typing is pending and the case is being closed. Practices with robust clinical record management that captures travel history, lab results, and organism identification reduce the frequency with which B64 needs to be applied.

When to use B64 vs. a more specific protozoal disease code

The ICD-10-CM coding principle is clear: code to the highest level of specificity supported by the documentation. B64 is appropriate only as a last resort after confirming that no more specific code applies. This call often falls first to primary care practices running GP clinic software, before a specialist referral confirms the causative organism. The table below maps the most common protozoal diseases to their specific codes, helping coders determine when B64 is and is not correct.

A practical note: if the record documents that the patient “has a protozoal infection” without organism identification, and the case is being closed or billed, B64 is appropriate. If testing is still outstanding, coders should document signs and symptoms rather than B64, and update when results arrive. The same specificity check applies to I61.x codes for intraparenchymal hemorrhage, where site-specific documentation determines the correct code.

Documentation requirements for B64

Before submitting a claim with B64, the chart needs to support the unspecified code with more than a diagnosis statement. Auditors reviewing an unspecified code want to see why a more specific code wasn’t available.

The clinical record should contain three things:

• An explicit statement that the organism was not identified. The note should state that the causative protozoan was not identified or documented, rather than leaving it to be inferred from silence.
• Evidence that organism testing was attempted or is pending. Lab orders, culture results, or a note that species identification wasn’t pursued show the coder didn’t skip a step that would have produced a specific code.
• The clinical rationale for closing the case as unspecified. A short note on why the case was coded and closed without organism identification – for example, the patient improved on empiric treatment before results returned – gives the claim something to point to if it’s audited.

Without these three elements, B64 looks like a shortcut rather than a residual code, which is exactly what payer audits flag. Coders reviewing a chart that’s missing organism-testing evidence should query the provider before billing B64, rather than defaulting to it because no other code seems to fit.

MS-DRG mapping for ICD-10 Code B64

For inpatient hospital billing, ICD-10-CM diagnosis codes map to Medicare Severity Diagnosis Related Groups (MS-DRGs), which determine reimbursement under the prospective payment system. According to the CMS ICD-10-CM/PCS MS-DRG Definitions Manual, B64 maps to the following MS-DRGs when listed as the principal diagnosis:

The actual MS-DRG assigned depends on the full claim: whether any secondary diagnoses qualify as CC or MCC conditions, and any procedures performed during the inpatient stay. MS-DRG groupings change annually with CMS updates, so always verify against the current CMS MS-DRG Definitions Manual version. The table above reflects the grouper logic consistent with MS-DRG v37.0+.

Billing teams that process inpatient claims should confirm their claims management workflows are updated to reflect the current fiscal year’s MS-DRG grouper tables when coding infectious disease admissions.

Index references for B64

The ICD-10-CM alphabetical index provides the lookup paths that direct coders from clinical terminology to B64. Coders looking up protozoal conditions in the index will find these entries pointing to B64:

• Disease, protozoal NOS: B64
• Infection, protozoan NOS: B64
• Protozoosis: B64

When the index entry for a condition routes to a more specific code within B50-B63, that code should always take precedence over B64. The NOS qualifier in the index indicates specifically that no further specification is available from the documentation. This convention isn’t unique to B64 – codes like M18.9 use the same unspecified designation, and coders should treat any NOS entry as a signal to query the provider before defaulting to it.

Code history and annual updates

B64 has been part of the ICD-10-CM system since the code set was implemented for U.S. payers. The table below summarizes its revision history and current status.

B64 has not been revised since ICD-10-CM implementation. It carries the same description, hierarchy position, and billable status it had at launch. Coders checking HIPAA-compliant coding requirements annually should verify status against the current ICD-10-CM tabular list each October, when new fiscal year editions take effect. The AAPC ICD-10-CM code reference updates annually and is a reliable secondary verification source alongside the CDC/NCHS official tool.

How Pabau supports accurate ICD-10 coding

Accurate application of unspecified protozoal disease codes like B64 depends on documentation quality at the point of care. When clinical records do not capture the causative organism, coders have no choice but to reach for the unspecified code, regardless of what testing may have been performed.

Pabau’s digital intake and clinical forms give practitioners a structured way to capture diagnosis detail, organism identification, and relevant travel or exposure history within the patient record. That documentation becomes the foundation for accurate ICD-10-CM assignment. For practices submitting HIPAA-covered claims, Pabau’s claims management software supports diagnosis recording and billing workflows that reduce the risk of unspecified codes appearing where specific ones should apply.

Coding teams managing infectious disease documentation can also benefit from structured compliance workflows that build organism identification queries into the standard review process, prompting clinicians to document specificity before the chart is closed. Practices concerned about HIPAA-compliant billing documentation will find that this level of clinical record specificity both reduces audit risk and improves coding accuracy across the full diagnosis code set.

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Conclusion

B64 is a narrow-use code. It covers one specific scenario: a confirmed protozoal infection with no identified organism. Applied correctly, it is a legitimate and billable ICD-10-CM diagnosis code for FY2026 HIPAA-covered transactions. Applied incorrectly, as a substitute for specific codes when organism identification is available, it invites audit queries and claim denials.

The decision rule is simple: consult the B50-B63 range first. If the organism is documented, a specific code exists. B64 applies only in the true residual case – the same logic that governs other chapter-ending unspecified codes, such as O90.9.

For practices looking to reduce unspecified code usage, strong documentation at the point of care is the most effective intervention. To see how Pabau supports diagnosis documentation and claims management in practice, book a demo with our team.

Continue your research

Curious how another ‘unspecified’ code in the same ICD-10-CM chapter works? A64 covers unspecified sexually transmitted disease and follows the same specificity-first rule as B64.

Coding another infectious disease diagnosis? I61.x codes for intraparenchymal hemorrhage show how site-specific documentation drives code selection.

Need a specific code to rule out before defaulting to unspecified? B79 covers trichuriasis, a related parasitic diagnosis in the same infectious disease chapter as B64.

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