ICD code G20.A1 – Parkinson's disease without dyskinesia

Key takeaways

G20.A1 is the billable ICD-10-CM code for Parkinson’s disease without dyskinesia, without mention of fluctuations, valid since October 1, 2023.

Two documented facts pick the G20 code. Dyskinesia moves it to G20.B, and OFF episodes move it to G20.A2.

Tremor is not dyskinesia, so a patient with a marked resting tremor can still be coded G20.A1.

Dementia plays no part in choosing A1 or A2. Report it as an additional F02 code, listed after the G20 code.

G20.C carries an Excludes1 note against G20.A1 and every G20.A and G20.B code, so it is never a fallback for confirmed Parkinson’s disease.

ICD-10 code G20.A1 means no dyskinesia and no OFF episodes

ICD-10 code G20.A1 covers Parkinson’s disease without dyskinesia, without mention of fluctuations. The CDC’s FY2027 ICD-10-CM tabular list also places Parkinson’s disease NOS here, so a plain “Parkinson’s disease” diagnosis lands on G20.A1. The patient has no documented dyskinesia and no documented OFF episodes.

Check the current-year descriptor in the CDC/NCHS ICD-10-CM browser tool before you load the code into a pick list or charge sheet.

Two questions pick the right Parkinson’s disease code

Since the FY2024 update, plain G20 is a category header and can’t go on a claim. Every Parkinson’s disease claim now needs one of four billable codes, and two yes-or-no questions get you there.

The matrix shows how the two answers combine.

Run them in this order as a quick Q&A with the note in front of you:

• Does the provider document dyskinesia? Yes takes you to G20.B. No, or no mention, keeps you in G20.A.
• Does the provider document fluctuations or OFF episodes? Yes gives you the “2” code. No mention gives you the “1” code.
• Is dementia or a mild neurocognitive disorder documented? It doesn’t change the G20 code. Add an F02 or F06.7- code after it.

G20.A1 vs G20.A2: OFF episodes make the difference

G20.A1 and G20.A2 both describe Parkinson’s disease without dyskinesia. The only split is fluctuation status. G20.A2’s inclusion term reads “with OFF episodes”, while G20.A1 reads “without mention of OFF episodes”.

What counts as a fluctuation

An OFF episode is a stretch when the benefit of levodopa fades and symptoms such as slowness, stiffness or tremor come back. It often happens before the next dose is due.

Look for it outside the assessment line, too. Providers often record it in the history or the medication plan. Typical phrases are “wearing off 45 minutes before each dose” or “adding a dose to cover OFF periods”.

When the note says “motor complications” or “wearing off” but never confirms fluctuations or OFF episodes, query the provider. Don’t pick G20.A2 on inference alone.

What doesn’t move the code

• Tremor, rigidity or slow movement. These are core features of the disease and fit every G20 code.
• Disease stage or duration. A Hoehn and Yahr stage or a 10-year history doesn’t change A1 to A2 on its own.
• Dementia or cognitive decline. Both are coded separately with F02 or F06.7-, as covered below.

Documented dyskinesia sends the code to G20.B

Dyskinesia here means involuntary writhing or jerking movements, most often caused by long-term levodopa. Tremor is part of the disease itself. A note that documents “resting tremor” therefore keeps the patient in G20.A.

Only explicit documentation of dyskinesia, or levodopa-induced dyskinesia, moves the code to G20.B1 or G20.B2. The G20.B subcategory also carries an Excludes1 note for drug induced dystonia (G24.0-), so those two can’t be reported together.

Dementia gets its own F02 code, not a different G20 code

Category G20 carries a “use additional code, if applicable” note for dementia and mild neurocognitive disorder. The F02 code adds the cognitive picture, and the G20 code stays the one the motor findings support.

The ICD-10-CM index shows it plainly. “Dementia, in, Parkinson’s disease” points to G20.A1, with F02.80 as the manifestation code. F02 is a manifestation code with a “code first” note, so it’s always listed after the G20 code.

The fourth character of F02 carries severity:

• F02.8- unspecified severity
• F02.A- mild
• F02.B- moderate
• F02.C- severe

The last character adds behavior: 0 for none, 1- for behavioral disturbance, 2 for psychotic disturbance, 3 for mood disturbance and 4 for anxiety. Severe dementia with psychotic disturbance is F02.C2, and moderate dementia with anxiety is F02.B4.

Two related diagnoses sit elsewhere. Mild neurocognitive disorder takes an F06.7- code. Neurocognitive disorder with Lewy bodies is G31.83, which the G21 secondary parkinsonism category lists as Excludes1.

Excludes notes for G20, straight from the tabular list

Here is what the FY2026 and FY2027 tabular lists print for this category. Notes that other sites attach to G20, such as Excludes1 for G21, G22 or G25, are not in the official text.

• G20 (category). Inclusion terms: hemiparkinsonism, idiopathic parkinsonism or Parkinson’s disease, paralysis agitans, primary parkinsonism or Parkinson’s disease. A use additional code note covers dementia (F02) and mild neurocognitive disorder (F06.7-). There is no Excludes1 or Excludes2 note at category level.
• G20.B (with dyskinesia). Excludes1: drug induced dystonia (G24.0-).
• G20.C (parkinsonism, unspecified). Excludes1: Parkinson’s disease NOS (G20.A1), Parkinson’s disease with dyskinesia (G20.B-), Parkinson’s disease without dyskinesia (G20.A-) and secondary parkinsonism (G21.-).

That last note matters most. G20.C is for a provider who documents parkinsonism without diagnosing Parkinson’s disease. It can’t be reported with G20.A1, and it’s not a fallback when dyskinesia or fluctuation status is unclear. Query the provider instead.

Drug-induced parkinsonism belongs in G21, with G21.11 for neuroleptic induced and G21.19 for other drugs. The choice between G20 and G21 rests on the cause the provider documents.

A worked example: One patient, three visits, three codes

Take an illustrative patient, a 68-year-old on carbidopa-levodopa, seen by the same neurologist over two years.

• Visit 1. The note confirms Parkinson’s disease with a right-hand resting tremor. It records no dyskinesia and no wearing off. Code G20.A1.
• Visit 2. The patient reports symptoms returning about 45 minutes before each dose. The neurologist documents OFF episodes and still no dyskinesia. Code G20.A2.
• Visit 3. The note adds peak-dose dyskinesia and documents moderate Parkinson’s disease dementia with no behavioral features. Code G20.B2, then F02.B0.

Each code matches what that day’s note says. A change from A1 to A2 to B2 is expected as the disease progresses, provided each note explains it.

What the note needs before you code G20.A1

A clean G20.A1 claim rests on four documented points. Each one maps to a word in the descriptor or a code you might add.

• A confirmed diagnosis. Outpatient coding rules in Section IV of the FY2027 ICD-10-CM Official Guidelines bar coding “possible” or “rule out” diagnoses. Code the presenting symptom, such as R25.1 tremor, until the provider confirms it.
• Dyskinesia status. State it as present or absent. Silence keeps you in G20.A, but an explicit “no dyskinesia” is stronger at audit.
• Fluctuation status. State whether the patient has OFF episodes or wearing off. This is the line that separates G20.A1 from G20.A2.
• Cognitive status, if impaired. Record the severity and any behavioral, psychotic, mood or anxiety features, so the right F02 code can be added.

How a G20.A1 claim moves from note to payment

1. The provider documents the visit. Dyskinesia, fluctuations and cognition are recorded alongside the exam and plan.
2. The coder reads the full note. They assign G20.A1 or its sibling, then any F02, R13.1- dysphagia or Z91.81 history of falling codes the note supports.
3. The claim links diagnosis to service. On the CMS-1500, G20.A1 goes in box 21, and the box 24E pointer ties it to the visit or therapy line.
4. The clearinghouse scrubs it. Code validity for the date of service is checked here, which is where a plain G20 is rejected.
5. The payer adjudicates. It checks the diagnosis supports the service, for example under a local coverage determination for therapy.
6. The remittance comes back. A denial arrives with a CARC, such as 11 for a diagnosis inconsistent with the procedure. Fix the cause, then resubmit.

Before you submit: A G20.A1 checklist

• The diagnosis is confirmed, not “possible” or “rule out”.
• No dyskinesia appears anywhere in the note, including the medication plan.
• No OFF episodes, wearing off or fluctuations appear in the history or plan.
• Plain G20 isn’t on the claim or in the pick list it came from.
• G20.C isn’t on the same claim as G20.A1.
• Any F02 code is listed after the G20 code, never first.
• The box 24E pointer links G20.A1 to the service it supports.

Common mistakes that get G20 claims denied or flagged

Most G20 problems trace back to a stale code list or a coder reading only the assessment. The table pairs each one with its fix. For broader workflows, see our guide to denial management in healthcare.

When a denial does land, the medical billing denial codes reference explains the CARC and RARC behind it.

G20.A1 in risk adjustment carries HCC 199

In the CMS-HCC V28 model, G20.A1 maps to HCC 199, Parkinson and Other Degenerative Disease of Basal Ganglia. G20.A2, G20.B1, G20.B2 and G20.C map to the same HCC, per the CMS risk adjustment model files.

Picking A1 over A2 won’t change the HCC, but it still matters. Risk adjustment audits compare the code with the note, and a mismatched subcode is an audit finding.

HCCs reset each calendar year. The diagnosis must be documented and coded from a face-to-face visit at least once a year to count. For the wider picture, see our guide to revenue cycle management.

How Pabau keeps G20.A1 claims clean

Most G20 errors start between the note and the claim form. The coder re-types a diagnosis, an old pick list still offers plain G20, or a dementia code lands in the wrong slot.

Pabau, the practice management and billing platform we build, pulls the diagnosis on the patient record onto the claim. Its ICD-10-CM lookup lists the five billable G20 codes with their descriptors. The claim won’t send until required fields, such as member ID and authorization code, are complete.

In the US, claims go through Claim.MD, with eligibility checks before the visit and ERA remittances posted back after it. A denied Parkinson’s claim then shows up on the record you already work from. See Pabau’s claims management software for the full workflow.

Send Parkinson’s disease claims with the right G20 code

Pabau carries the diagnosis from the patient record onto the claim and checks required fields before it sends. US claims go through Claim.MD, with eligibility checks and ERA remittances.

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Conclusion

G20.A1 is the right code far more often than its long descriptor suggests. It covers any confirmed Parkinson’s disease without dyskinesia or OFF episodes, including Parkinson’s disease NOS.

If you change one habit, make it reading the medication plan before you code. That’s where wearing off and dyskinesia tend to hide, and they’re the only two findings that move the code. Leave dementia to its own F02 line.

Book a demo to see how Pabau carries the right Parkinson’s code from the note to a clean claim.

Continue your research

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Looking for a primer on the full billing cycle behind neurology claims? What is medical billing explains the end-to-end process from encounter to payment posting.

Want to understand 837 file submission for ICD-10 diagnosis codes? 837 file guide covers how diagnosis codes are structured in electronic claim submissions.

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