William Giesen, M.D.
NPI 1124104724 · Internal Medicine Physician · Darien, CT
Enrolled with Medicare as Practitioner - Internal Medicine · source: CMS PECOS Public Provider Enrollment (2026.04.01)
William Giesen, M.D. is a Internal Medicine Physician provider in Darien, CT (NPI 1124104724).
Provider details
- NPI number
- 1124104724
- Entity type
- Individual
- Primary specialty
- Internal Medicine PhysicianAllopathic & Osteopathic Physicians
- Credential
- M.D.
- Sex
- Male
- Sole proprietor
- Yes
- License
- CT023721 (CT)
Specialties & taxonomies
Allopathic & Osteopathic Physicians
Profile Score
A profile completeness & documented-activity index — NOT a quality, outcomes, or patient-satisfaction rating.
- Registry tenure28% of score
19 yrs in NPPES registry
- Profile completeness28% of score
100% of core fields populated
- Practice footprint17% of score
1 documented practice location
- Medicare service volume (vs. specialty peers)28% of score
894 Medicare services, 2024 (ranked within specialty)
Medicare volume is ranked against active providers in the same specialty. Signals a provider has no public data for (e.g. Medicare claims) are left out of the score entirely — never counted as zero — so providers without that data are not penalized. See the full formula and weights in our methodology.
What this provider does (Medicare)
Based on Medicare fee-for-service claims, 2024; not all care is captured.
| Service (HCPCS) | Medicare services | Typical Medicare-allowed (2024) |
|---|---|---|
| Established Patient Office Or Other Outpatient Visit With Low Level OD Decision Making, If Using Time, 20 Minutes Or More99213 | 186 | $95 |
| Insertion of Needle Into Vein for Collection of Blood Sample36415 | 153 | $9 |
| Urinalysis, Manual Test81002 | 114 | $3 |
| Routine Electrocardiogram (ecg) Using at Least 12 Leads With Interpretation and Report93000 | 107 | $15 |
| Test to Measure Expiratory Airflow and Volume94010 | 99 | $29 |
| Established Patient Office Or Other Outpatient Visit With Moderate Level of Decision Making, If Using Time, 30 Minutes Or More99214 | 97 |
What this provider prescribes (Medicare Part D)
Top drugs by Medicare Part D claim volume, 2024 · 32 distinct drugs on file.
| Drug | Part D claims (2024) | Beneficiaries |
|---|---|---|
| Atorvastatin Calcium | 256 | 58 |
| Losartan Potassium | 152 | 34 |
| Amlodipine Besylate | 152 | 32 |
| Hydrochlorothiazide | 98 | 17 |
| Zolpidem Tartrate | 80 | 16 |
| Escitalopram Oxalate | 69 | — |
| Azithromycin | 40 |
Practice location
Mailing address
Registry dates
- Enumerated
- 2006-10-27
- Last updated
- 2016-05-09
Frequently asked questions
- What is William Giesen's NPI number?
- William Giesen's National Provider Identifier (NPI) is 1124104724, a unique 10-digit ID issued by CMS for use in standard healthcare transactions.
- What is William Giesen's specialty?
- William Giesen is listed in the NPPES registry with a primary specialty of Internal Medicine Physician (Allopathic & Osteopathic Physicians).
- Where is William Giesen located?
- William Giesen's practice location on file with NPPES is 1500 Post Rd., Darien, CT 06820.
- What is William Giesen's phone number?
- The practice phone number listed in the NPPES registry for William Giesen is (203) 655-2516.
- Does William Giesen accept Medicare?
- William Giesen is approved to bill Medicare according to CMS PECOS enrollment data (2026.04.01). Confirm current participation with the provider before scheduling.
- Is William Giesen's NPI active?
- Yes — William Giesen's NPI (1124104724) is active in the NPPES registry as of the last update on 2016-05-09.
Source: CMS NPPES Data Dissemination (June 2026). NPI Central is not affiliated with CMS. Data is provided as-is from the public registry.