Seasonal variation in diagnoses and visits to family physicians.
BACKGROUND
Practice-based research networks (PBRNs) replicating the National Ambulatory Medical Care Survey (NAMCS) must sample more than 1 year to account for presumed seasonal variation in illnesses. This study evaluated the effects of seasonality on diagnoses within NAMCS family physician data.


METHODS
Using combined data from the 1995-1998 NAMCS, diagnostic clusters that accounted for more than 1% of total visits were analyzed for seasonality. Seasons were coded categorically as dummy variables with summer as the reference category. A logistic regression was performed with each diagnosis as an outcome on the full data. To examine the ability of alternative sampling strategies to replicate the full year of data, a simulation study was carried out drawing 50 samples of 1,000 visits each for winter-summer and spring-fall sampling periods.


RESULTS
We found 23 diagnostic clusters that had a frequency more than 1%, of which 10 had seasonal variations (P < or = .001), primarily between winter and summer. If sampling were restricted to spring, the diagnostic clusters of pregnancy and coronary artery disease would account for less than 1% of visits. All other diagnostic clusters, though changing rank order, would account for more than 1% if sampled in a single quarter. In the simulated sampling strategy, visit prevalence dropped below 1% for at least 1 diagnosis in 24 of 50 samples in spring-fall compared with 20 of 50 samples for winter-summer (P > .20).


CONCLUSIONS
There is little seasonal variation in the 23 diagnoses that occur in more than 1% of visits to family physicians. There is, however, important seasonal variation in the rank order of these diagnoses. A sampling strategy that uses any quarter of the year but spring (March, April, May) could be used to understand what diagnoses are frequently seen within a PBRN.
Pace
Wilson D
WD
Department of Family Medicine, University of Colorado Health Sciences Center, Aurora, Colo 80045-0508, USA. wilson.pace@uchsc.edu
Dickinson
L Miriam
LM
Staton
Elizabeth W
EW
D000328:Adult; D000368:Aged; D016000:Cluster Analysis; D004813:Epidemiology; D005194:Family Practice / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D038801:International Classification of Diseases; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D009819:Office Visits / Q000706:statistics & numerical data*; D015995:Prevalence; D012621:Seasons; D014481:United States / Q000453:epidemiology