# Presentation: 134089

## Using past visit information to enhance analysis of National Ambulatory Medical Care Survey (NAMCS) data

- Session 25
- July 13, 2004
- 10:30-noon

## Session overview

- Introduction and methodology for using past visit information – *Catharine Burt*
- Example of analysis of quality of care for hypertension – *Esther Hing*

## Using past visit information to enhance analysis of ambulatory care from encounter data

***                      ******Catharine W. Burt , Ed.D. ***
***Chief, Ambulatory Care Statistics Branch***

***July 13, 2004 Data Users Conference***

## Topics

**Introduction**
**Brief survey background**
**Past visit item**
**Re-weighting methodology**
**Analysis using re-weighting**
**Summary**

## Encounter vs. population-based surveys that measure ambulatory care utilization

**Population-based **
**National Health Interview Survey (NHIS)**
**National Health and Nutrition Examination Survey (NHANES)**
**Medical Expenditure Panel Survey (MEPS)**
**Encounter-based **
**National Ambulatory Medical Care Survey (NAMCS)**
**National Hospital Ambulatory Medical Care Survey (NHAMCS)**
**       **

## Slide 6

## Rate of visits per population by patient age: NAMCS, NHAMCS-OPD, 2001

## Desired goals from adding number of past visits to encounter data

- Describe patterns of care by frequency of visiting the doctor
- Provide more information about patients from encounter-level data
- Better describe quality of care to patients vs. describing content of encounter

## NAMCS and NHAMCS background 

**NAMCS   **
**Fielded 1973-1981, 1985, 1989-present**
**3-stage sample**
**PSUs – **
**   ****physicians – **
**   ****visits during 1 week**
**NHAMCS **
**Fielded annually since 1992**
**4-stage sample**
**PSUs – **
**	****hospitals – **
**	****ED/OPD clinics – **
**   ****visits during 4 weeks**

- SEs

- rates

- percents

- drugs

- providers

- visits

- sample

- records

## Scope of the NAMCS

- Physicians must be:
- Primarily engaged in office-based, patient care
- Non-federally employed
- Not in anesthesiology, radiology, or 	pathology

## In-Scope Visits for NAMCS

**Basic unit of sampling is the physician-patient visit **
**Visit must occur in a physician’s office**
**Visits must be for medical purposes**
**Out-of-scope contacts include:**
**Visits for administrative purposes**
**House calls, phone calls, emails, visits in hospital setting unless physician has private office there**

## In-Scope NAMCS Locations 

**Freestanding private solo or group practice**
**Freestanding clinic/urgicenter**
**Neighborhood medical and mental health centers**
**Privately operated clinics**
**Non-Federal government clinic**
**Health maintenance organization**
**Federally qualified health center**
**Faculty practice plan**

## Out-of-Scope NAMCS Locations

- Hospital ED’s and OPD’s
- Ambulatory surgicenter
- Institutional setting (schools, prisons)
- Industrial outpatient facility
- Federally operated clinic
- Laser vision surgery

## Scope of the NHAMCS

- Basic unit of sampling is patient visit
- Emergency and outpatient departments of non-federal, general and short-stay hospitals
- Not Federal, military, or Veterans Administration facilities
- Located in 50 states and D.C.

## Items Collected

**Patient characteristics **
**Age, sex, race, ethnicity**
**Visit characteristics**
**Source of payment, continuity of care, reason for visit, diagnosis, treatment, medications ordered or provided**
**Provider characteristics**
**Physician specialty, hospital ownership**
**Drug characteristics**
**Therapeutic class, composition, control status, ingredients, Rx or OTC**

## Slide 16

## Slide 17

## Percent distribution for number of past visits within l2 months: NAMCS, 2001

## Percent distribution of past visit item by PCP status: NAMCS 2001

## Percent of visits where the patient made 6 or more visits in past year by patient age: NAMCS 2001

## Percent of visits to patient's primary care provider (PCP) by number of past visits within last l2 months: NAMCS, 2001

## Drug mention rate by past visits: NAMCS, 2001

## Drug mention rates by number of past visits for selected therapeutic classes: NAMCS, 2001

## Drug mention rates by number of past visits for selected therapeutic classes: NAMCS, 2001

## Re-weighting methodology

- Assumptions and definitions
- Dividing the visit weight by number of visits within year provides a measure of patients
- Patient is relation between person and sampled doctor
- One person can be different patients to different doctors
- Assumes previous visits by same patient have similar visit characteristics

## 3 patients making a total of 7 visits

- V V V V

- v v

- V

## 4/7

- V V V V

**4/7**

**2/7**

**1/7**

- v v

- Probability of selecting a visit

- V

## 4/7

- V V V V

- 2/7

- 1/7

**1**

**.5**

**.25**

- Multiplicity factor

- V

- v v

## Creation of a re-weighting factor

| Item categories | Past visits | Annual visits | Interval midpoint | Viz ratio |
| --- | --- | --- | --- | --- |
| New | 0 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 |
| 1-2 | 1-2 | 2-3 | 2.5 | .4 |
| 3-5 | 3-5 | 4-6 | 5 | .2 |
| 6+ | 6+ | 7+ | 8 | .125 |

## Patient weight = Visit weight * Viz ratio 

Visit weight     →  880,487,000 visits
				       ± 33,373,000

Patient weight →  322,739,000 patients
		          ± 13,073,000

- Visit weight     →  880,487,000 visits
- 				       ± 33,373,000
- Patient weight →  322,739,000 patients
- 		          ± 13,073,000

## Re-weighting example

| Annual visits | Interval midpoint | Visit weight | Viz ratio | Patient weight |
| --- | --- | --- | --- | --- |
| 1 | 1 | 10 | 1 | 10 |
| 2-3 | 2.5 | 20 | .4 | 8 |
| 4-6 | 5 | 40 | .2 | 8 |
| 7+ | 8 | 40 | .125 | 5 |

**Sum =  110 visits                   31 patients**

## Analysis using re-weighting

- Comparison of visits and patients
- Analysis of patients

## Comparison of distributions for visits and patients: NAMCS 2001

**Visits**

**Patients**

## Percent distribution for people making any health care visits by number of visits made in one year: NHIS, 1999-2000

**1-3**

**4-9**

**10+**

- Number of visits

- 0

- 10

- 20

- 30

- 40

- 50

- 60

- Percent of persons

- Rate of persons making no health care visit was 17.5.

## Rate of visits per population by patient age: NAMCS, NHAMCS-OPD, 2001

## Components of utilization rates

- Rate of visits per population =
- Rate of visits per patient
-            * **multiplied by*
- Rate of patients per population

## Rate of visits per patient and patients per population by patient age: NAMCS, NHAMCS-OPD, 2001

## Slide 38

## Percent of visits and patients where the patient made 7 or more visits in the year by patient age: NAMCS 2001

**Visits**

**Patients**

## Percent of visits and patients where the patient made only 1 visit in the year by patient age: NAMCS 2001

**Visits**

**Patients**

## Rate per 100 pop of patients to same provider by type of setting and patient age: NAMCS, NHAMCS-OPD, 2001

## Rate per 100 pop of patients by number of visits within a year to same provider and PCP status: NAMCS, 2001

**1**

**2-3**

**4-6**

**7+**

- Number of visits

- 0

- 10

- 20

- 30

- 40

- 50

- 60

**Number of patients per 100 pop**

**PCP**

**Non-PCP**

## Rate per 100 pop of patients by number of visits within a year to PCP: NAMCS and NHAMCS-OPD, 2001

- 0

- 10

- 20

- 30

- 40

- 50

**Number of patients per 100 pop**

**1**

**2-3**

**4-6**

**7+**

## Incidence of tobacco use cessation counseling at PCP visits: NAMCS & NHAMCS-OPD, 2001

## Percent of visits by tobacco users to their PCP with tobacco cessation counseling by number of past visits within l2 months: NAMCS & NHAMCS-OPD, 2001

## Public health example

**Women at risk for bearing children with birth defects **
**Levothyroxine (evidence of hypothyroid)**
** ****2.6 million visits**
** ****2.3 million patients**
** ****Accutane**
**397 thousand visits**
**272 thousand patients**

## Comparison of drug mention rates for visits and patients

| Drug class | Visit DMR | Patient DMR | Ratio |
| --- | --- | --- | --- |
| CNS | 13 | 10 | 1.35 |
| Cardio | 22 | 17 | 1.29 |
| Pain relief | 18 | 17 | 1.10 |
| Antimicrobial | 13 | 12 | 1.08 |

- DMR is number of drugs per 100 visits/patients.

## Ratio of drug mention rates based on visits and patients: NAMCS, 2001

- ,

- ,

- ,

- ,

**CNS**

**Cardio**

**Pain relief**

**Antimicrobial**

- .

- 0.6

- 0.8

- 1

- 1.2

- 1.4

- 1.6

- 1.8

## Summary

**Past visits items provides depth to analysis of ambulatory care utilization**
**Visit records may be re-weighted to provide patient-level estimates**
**Re-weighted distribution more closely resembles population-based estimates**
**No change in sampling variance estimation procedure other than using the new weight **
**Caveats**
**Assumption of similar characteristics is not applicable to all analytical variables**
**Still is not equivalent to person-level estimates**

## Information available on the Internet

- NCHS website is _[www.cdc.gov/nchs](http://www.cdc.gov/nchs)_
- National Health Care Survey website
- Ambulatory Health Care Data (AHCD)
- Join our LISTSERV

**Notes:**
Added the acromyns for NNHS and NHHCS – replace slide # 53