What is ICD-10 code B59?

B59 is the ICD-10-CM diagnosis code for Pneumocystosis. A serious lung infection caused by Pneumocystis, a fungal organism, that primarily affects people with weakened immune systems. It belongs to the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99). B59 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. B59 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. B59 is a payment HCC under the CMS risk adjustment model. Under the CMS-HCC V28 risk adjustment model it maps to Opportunistic Infections with a community, non-dual, aged base RAF weight of 0.381. Under the older CMS-HCC V24 model it maps to Opportunistic Infections at RAF 0.424. V28 reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. HCC Buddy maintains the V28 and V24 mapping, RAF weight, and MEAT documentation criteria for B59 sourced directly from the CMS-HCC model files.

Yes. B59 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. B59 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for B59?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For B59, V28 maps to Opportunistic Infections at RAF 0.381 while V24 mapped to Opportunistic Infections at RAF 0.424. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting B59?

This diagnosis is commonly associated with HIV/AIDS; verify and code the underlying immunocompromised condition Specify the type of pneumonia (PCP) and whether it is the initial diagnosis or recurrent Because B59 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

B59: Pneumocystosis — ICD-10 | Opportunistic Infections

ICD-10-CM Code View

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Code lookupB59Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Certain infectious and parasitic diseases (A00-B99) / Protozoal diseases (B50-B64)

B59

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Pneumocystosis

A serious lung infection caused by Pneumocystis, a fungal organism, that primarily affects people with weakened immune systems.

Buddy Insight

Pneumocystosis (Pneumocystis jirovecii pneumonia, formerly called PCP) is one of the most common and best-recognized AIDS-defining opportunistic infections.

CMS-HCC V28

Mapped

HCC 6

Opportunistic Infections

RAF 0.381

CMS-HCC V24

Mapped

HCC 6

Opportunistic Infections

RAF 0.424

ACA/HHS

Mapped

HCC 6

HHS-HCC 6

Varies by metal level

ESRD/PACE

Mapped

HCC 6

Opportunistic Infections

RAF 0.076

RXHCC

Mapped

HCC 5

Opportunistic Infections

RAF 0.468

Code Book Path

Official

B5Protozoal diseases (B50-B64)

B59Pneumocystosis

Inclusion Terms

Official

Pneumonia due to Pneumocystis carinii
Pneumonia due to Pneumocystis jirovecii

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for B59 in this effective period.

Related Child Codes

Official

ICD-10-CM does not list child codes under B59 for this display context.

Includes

Official

ICD-10-CM does not list Includes notes for B59 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for B59 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for B59 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for B59 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for B59 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Bronchoalveolar lavage or induced sputum with positive Pneumocystis jirovecii stain (GMS, DFA, or PCR)

Chest imaging showing bilateral ground-glass opacities (classic pattern)

Arterial blood gas or pulse oximetry showing hypoxemia

CD4 count documented (typically <200 cells/microL) and HIV status

MEAT Support

HCC Buddy guidance

Bronchoalveolar lavage or induced sputum with positive Pneumocystis jirovecii stain (GMS, DFA, or PCR)

Chest imaging showing bilateral ground-glass opacities (classic pattern)

Arterial blood gas or pulse oximetry showing hypoxemia

CD4 count documented (typically <200 cells/microL) and HIV status

Audit Caution

HCC Buddy guidance

Coding as unspecified pneumonia (J18.9) when Pneumocystis has been confirmed — this loses both clinical accuracy and HCC capture

Not coding B20 (HIV disease) as the underlying condition — B59 alone does not capture the immune deficiency

Confusing prophylaxis for Pneumocystis (using trimethoprim-sulfamethoxazole preventively) with active disease requiring the B59 code

Missing the diagnosis in non-HIV immunocompromised patients (transplant, chemotherapy) where PCP also occurs

Common Mistakes

HCC Buddy guidance

B58.3 (Pulmonary toxoplasmosis) - similar bilateral pneumonia in AIDS; bronchoalveolar lavage differentiates

J18.9 (Pneumonia, unspecified) - Pneumocystis pneumonia must be coded specifically with B59, not as unspecified pneumonia

B20 (HIV disease) - HIV should be coded in addition to B59, not instead of it

J15.9 (Unspecified bacterial pneumonia) - PCP is not bacterial; it is caused by a fungal organism

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is B59 an HCC code?

Yes. B59 maps to Opportunistic Infections under the CMS-HCC V28 risk adjustment model (and Opportunistic Infections under V24).

HCC Category Mapping

V28HCC 6, Opportunistic Infections

0.381

V24HCC 6, Opportunistic Infections

0.424

ESRDHCC 6, Opportunistic Infections

0.076

RxHCCHCC 5, Opportunistic Infections

0.468

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.

Work B59 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for B59

For B59to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed B59 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

B59 is the ICD-10-CM diagnosis code for pneumocystosis. A serious lung infection caused by Pneumocystis, a fungal organism, that primarily affects people with weakened immune systems. B59 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering protozoal diseases (b50-b64).

Under the CMS-HCC V28 risk adjustment model, B59 maps to Opportunistic Infections (HCC 6) with a community, non-dual, aged base RAF weight of 0.381. Under the older V24 model, B59 mapped to the same category but with a base RAF weight of 0.424, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.

This diagnosis is commonly associated with HIV/AIDS; verify and code the underlying immunocompromised condition. Because B59 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for B59 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•This diagnosis is commonly associated with HIV/AIDS; verify and code the underlying immunocompromised condition
•Specify the type of pneumonia (PCP) and whether it is the initial diagnosis or recurrent

Clinical Significance

Pneumocystosis (Pneumocystis jirovecii pneumonia, formerly called PCP) is one of the most common and best-recognized AIDS-defining opportunistic infections. It remains the most frequent cause of respiratory failure in untreated HIV patients with CD4 counts below 200. Despite effective prophylaxis, it continues to occur in undiagnosed HIV patients and those non-adherent to treatment, carrying 10-20% mortality even with appropriate therapy.

Documentation Requirements

✓Bronchoalveolar lavage or induced sputum with positive Pneumocystis jirovecii stain (GMS, DFA, or PCR)
✓Chest imaging showing bilateral ground-glass opacities (classic pattern)
✓Arterial blood gas or pulse oximetry showing hypoxemia
✓CD4 count documented (typically <200 cells/microL) and HIV status
✓Treatment documented: trimethoprim-sulfamethoxazole for 21 days, adjunctive corticosteroids if PaO2 <70
✓LDH level (typically elevated; useful for prognostication)

Commonly Confused Codes

•B58.3 (Pulmonary toxoplasmosis) - similar bilateral pneumonia in AIDS; bronchoalveolar lavage differentiates
•J18.9 (Pneumonia, unspecified) - Pneumocystis pneumonia must be coded specifically with B59, not as unspecified pneumonia
•B20 (HIV disease) - HIV should be coded in addition to B59, not instead of it
•J15.9 (Unspecified bacterial pneumonia) - PCP is not bacterial; it is caused by a fungal organism

Code Hierarchy

└B59Pneumocystosis

B59 ICD-10-CM Code: Pneumocystosis

HCC Buddy Code Card

B59

Is B59 an HCC code?

HCC Category Mapping

MEAT Criteria for B59

Get the V28 RAF + MEAT cheat sheet

What This Code Means

Coding Tips

Clinical Significance

Documentation Requirements

Commonly Confused Codes

Code Hierarchy

Work B59 in HCC Buddy