Frederick Stack, MD
Frederick Stack, MD is an Internal Medicine Physician in Sioux City, IA, registered under NPI 1548256977. This provider registers 2 separate taxonomy codes, which usually means they practise across more than one recognised specialty. NPPES records them as a sole proprietor, meaning the practice is not incorporated as a separate legal entity.
Getting there
The practice address on file is 801 5TH ST, Sioux City, IA 51101. We have not yet mapped what surrounds this office — parking, transit, and nearby pharmacies are added as our data collection reaches each location.
Credentials and Medicare
| Licence | State | Specialty |
|---|---|---|
| MD-46368 | IA | Internal Medicine Physician primary |
| 91977 | MT | Internal Medicine Physician |
| 24459 | AZ | Specialist |
Licences are recorded in 3 states: IA, MT, AZ. Providers licensed in several states often practise near a state border or deliver telehealth across state lines.
- Status
- Approved
- Provider type
- Practitioner - Internal Medicine
- PAC ID
- 9638149230
Enrolled in Medicare 4 times, across 4 states:
| State | Enrollment ID | Provider type |
|---|---|---|
| Arizona | I20040730000095 | Practitioner - Internal Medicine |
| New Mexico | I20200511003250 | Practitioner - Internal Medicine |
| Montana | I20210122002608 | Practitioner - Internal Medicine |
| Maine | I20230810000561 | Practitioner - Hospitalist |
Medicare enrolment is recorded as approved under the provider type Practitioner - Internal Medicine. There are 4 separate enrolment records across 4 states (AZ, NM, MT, ME). Multiple enrolments usually mean the provider bills Medicare from more than one practice location, or has kept an earlier enrolment after moving. Each record carries its own enrolment ID, listed above. Being enrolled means the provider can bill Medicare; it does not tell you whether they are currently accepting new Medicare patients. Call the office to confirm.
Licensed in 3 states
This provider holds licences in 3 states: Arizona, Iowa and Montana. The practice address on file is in Sioux City, IA, but a licence in another state usually means they see patients there too — either at a second location or by telehealth. If the address above is far from you, it is worth calling to ask whether they practise closer to where you live.
- Licence
- MD-46368
- Specialty
- Internal Medicine Physician
Licence numbers are self-reported to NPPES at registration. They do not confirm a licence is currently active, in good standing, or valid for the specialty shown — only the state board can tell you that, which is why we link to each one.
What Internal Medicine covers
A physician who provides long-term, comprehensive care in the office and the hospital, managing both common and complex illness of adolescents, adults and the elderly. Internists are trained in the diagnosis and treatment of cancer, infections and diseases affecting the heart, blood, kidneys, joints and digestive, respiratory and vascular systems. They are also trained in the essentials of primary care internal medicine, which incorporates an understanding of disease prevention, wellness, substance abuse, mental health and effective treatment of common problems of the eyes, ears, skin, nervous system and reproductive organs. NUCC Health Care Provider Taxonomy, code 207R00000X
All registered taxonomy codes
This provider registers 3 taxonomy codes. The primary code is what appears on most claims; the others record additional recognised specialties.
| Code | Specialty | Licence | State | |
|---|---|---|---|---|
| 207R00000X | Internal Medicine Physician | MD-46368 | IA | primary |
| 207R00000X | Internal Medicine Physician | 91977 | MT | |
| 174400000X | Specialist | 24459 | AZ |
The taxonomy records how the provider classifies their own practice. It does not confirm which conditions they currently treat, whether they are accepting new patients, or which insurance they take — call the office for that.
Full NPPES record
An NPI-1 identifies a single person. It stays with the clinician for life, even if they move practice or change state, which is why the address on file can lag behind where they actually work today.
| NPI | 1548256977 valid check digit |
|---|---|
| Entity type | Individual (NPI-1) |
| Enumeration date | Sep 21, 2005 |
| Last updated | Jan 28, 2026 |
| Status | Active |
| Sole proprietor | Yes |
| Credential | MD |
| Practice address |
801 5TH ST Sioux City, IA 51101 (712) 279-2010 Fax (712) 279-2034 |
| Mailing address |
6909 EAST RAY ROAD #15-114 Chandler, AZ 85226 |
| Other identifiers |
436693002
· Medicaid (AZ) AZ0866740 · Other (AZ) 110208816 · Other (AZ) |
Number check
How the check works, step by step
Every NPI carries a Luhn check digit — the same scheme used on credit cards. CMS prefixes the number with 80840 before computing it, which marks it as a US health identifier.
| Digit | 8 | 0 | 8 | 4 | 0 | 1 | 5 | 4 | 8 | 2 | 5 | 6 | 9 | 7 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Doubled? | — | ×2 | — | ×2 | — | ×2 | — | ×2 | — | ×2 | — | ×2 | — | ×2 |
| Value | 8 | 0 | 8 | 8 | 0 | 2 | 5 | 8 | 8 | 4 | 5 | 3 | 9 | 5 |
Sum = 73 · next multiple of 10 = 80 · check digit = 7 · actual last digit = 7
A valid check digit only means the number is well-formed. It does not mean the provider is licensed, active, or in good standing — those are separate questions answered by the sections above.