Prevalence of Estimated GFR Reporting Among US Clinical Laboratories

Study Population and Sampling 

On November 22, 2005, NKDEP obtained the Clinical Laboratory Improvement Amendments (CLIA) database from the Centers for Medicare & Medicaid Services, which includes all laboratories that hold or are seeking 1 of 4 types of certification required to perform laboratory tests on humans in the United States. The file includes laboratory/facility type (eg, community clinic and hospital), testing specialties and subspecialties, related annual test volumes, and other information. Of the nearly 200,000 laboratories in the CLIA database, we identified 20,532 that met the 2 inclusion criteria for this study: (1) possession of (or applying for) a Certificate of Compliance or Accreditation, and (2) a specialty in chemistry with a subspecialty in routine chemistry testing.

These 20,532 laboratories represent the universe of those that could be reporting eGFR and constitute the sample frame for this study. A sample was designed to allow results to be generalized to US-based clinical laboratories. The sample design maximized the precision of the estimated proportion of laboratories reporting eGFR for the national sample, as well as for 6 meaningful laboratory categories. The first 4 categories are those with the largest number of laboratories of 27 unique types of laboratories/facilities identified in the CLIA database. The 4 categories, accounting for 84.6% of all those that met the inclusion criteria, are physician office (7,627), hospital (6,574), independent (which traditionally conduct high test volumes; 2,174), and community clinic (986) laboratories. Samples were drawn from each of these 4 laboratory/facility types. The fifth category includes a small number of individual laboratories (28 in all) representing 3 laboratory types—public health (14), health fair (11), and insurance (3)—that account for 60.5% of the total volume of routine chemistry tests performed nationally. The 28 were combined to form a single group of “high-volume” laboratories that would be examined separately. The sixth or “other” category represents the 3,143 laboratories from the remaining 20 laboratory/facility types. As a result, a total of 6 laboratory-type strata were created for sampling purposes. Table 1 lists brief descriptions of the different types of laboratories.

Table 1. Descriptions of Types of Laboratory Facilities

	Stratum (no.)	Description of Type of Laboratories	Why Selected as a Stratum
	Physician office (7,627)	Tests performed in physician office setting; results typically shared during visit Practices are often small, but can be large (2 or 3 to 200 providers) May conduct only rapid tests or operate laboratories like those in hospitals	Highest number of facilities that met inclusion criteria Patient population with CKD risk factors (diabetes and hypertension)
	Hospital (6,574)	Tests performed include those needed in emergency situations and those done in high enough volume to warrant acquisition of necessary equipment May be segmented by chemistry, pathology, and other specialty divisions Usually proportionate in size to the population it serves; generally used by all inpatients at particular hospital and many outpatients seen by physicians with offices in hospital Send some tests to reference laboratories if demand is low	Second-highest number of facilities that met inclusion criteria
	Independent (2,174)	Blood chemistry analyses and urinalyses are some of most frequently requested tests Generally conduct high routine and specialty test volumes; often operate all day/week Private commercial facilities, including 2 largest national providers, Quest Diagnostics and LabCorp; at least 35 other companies exist Also known as reference laboratories; most tests requested from physician offices and hospitals	Third-highest number of facilities that met inclusion criteria
	Community clinic (986)	Laboratories that are on site at community clinics Laboratories perform tests on samples drawn from patients on site; some samples sent to reference laboratories for testing Patients typically get results during follow-up visits	Serve populations disproportionately affected by CKD risk factors Fourth-highest number of facilities that met inclusion criteria (excluding CLIA “other” category)
	Health fair/insurance/public health (28)	Health fair laboratories are set up as part of a health fair, health assessment, or health risk reduction program; can include lipid testing, measurement of prostate-specific antigen, and comprehensive chemistry panels. Usually operated by a clinical laboratory under special permit and must follow strict procedural and management guidelines Insurance laboratories perform tests required by insurance companies to determine whether to extend coverage or pay a claim Public health laboratories typically function to safeguard communities through monitoring communities for pathogens that spread through food/people/animals, testing to detect and monitor newly emerging infectious diseases, and so on	Exceptionally high mean annual routine chemistry volumes (range, 524,460 to 1,658,704)
	Other (3,143)	Mix of remaining laboratory types: ambulatory surgery center, comprehensive outpatient rehabilitation facility, ancillary testing site in health care facility, end-stage renal disease dialysis facility, health maintenance organization, home health agency, hospice, industrial, intermediate-care facility for mentally retarded, mobile laboratory, pharmacy, school/student health service, skilled nursing facility/nursing facility, other practitioner, tissue band/repositories, blood banks, rural health clinics, federally qualified health centers, ambulance, and other	Catch all for remaining laboratories, including CLIA “other” category

Abbreviations: CKD, chronic kidney disease; CLIA; Clinical Laboratory Improvement Amendments.

A minimum sample size was calculated for each of the 6 strata using a proportional sampling approach. Based on the 2005 CAP survey, the expected proportion of laboratories reporting eGFR was set to 0.20 and the desired level of precision was set to ±2.0% with 95% confidence and corrected for each stratum's finite size. Taking into account the design effect of the stratified sample (design effect = 1.24), the overall precision of the weighted national estimate was expected to provide a level of precision of ±1%. Planning for an 80% response rate, according to Office of Management and Budget requirement, the sample size randomly drawn from each of the sampled strata was 1,599 physician office, 1,557 hospital, 1,125 independent, and 751 community clinic laboratories, all 28 high-volume laboratories (a census of all 3 laboratory types, not a sample), and 1,290 other. The total sample was 6,350 laboratories.

Instrument Development and Data Collection 

NKDEP developed a 10-item questionnaire (Table 2) for both paper-and-pencil and web administration modes. The questions are based in part on survey questions used by CAP and 2 state departments of health that were known to have asked laboratories about eGFR-reporting practices. The paper-based instrument was pretested by laboratory professionals before implementation, and feedback was used to fine-tune the wording of questions and response selections. Although NKDEP was most interested in determining an estimate for the prevalence of eGFR reporting, the survey also provided an opportunity to ask questions related to the NKDEP reporting-related recommendations (see introduction). The web version was pretested by communication professionals to ensure that it was easy to use. On October 20, 2006, a cover letter and questionnaire, including a postage-paid return envelope, were mailed to the sample of 6,350 clinical laboratories. Addressed to laboratory directors/managers, recipients were requested to either complete and return the paper questionnaire or log on to the NKDEP website to access the electronic questionnaire. One week later, a reminder postcard was mailed to the entire sample.

Table 2. Survey Questions for NKDEP Study to Assess the Prevalence of eGFR Reporting

	Item No.	Question	Response Choices
	1	Does your laboratory report serum creatinine values for adults (≥18 y)?	Yes, No, Not sure
	2	How does your laboratory report serum creatinine values?	mg/dL, μmol/L
	3	To how many decimal places do you report the creatinine result?	None, 1, 2
	4	How many serum creatinine tests did your laboratory perform in 2005?	Fill in the blank
	5	Does your laboratory EVER report eGFRs with serum creatinine determinations?	Yes, No, Not sure
	6	Is your laboratory currently considering reporting eGFR with serum creatinine determinations?	Yes, No, Not sure
	7	Under what circumstances does your laboratory report eGFR?	With ALL measured serum or plasma creatinine determinations, Only when specifically requested, Other: please specify
	8	Which estimating equation do you use for your reports?	4-variable MDRD Study, 6-variable MDRD Study, Cockcroft-Gault, Not sure, Other: please specify
	9	When reporting eGFR, at what point do you assign a “greater than” (>) value?	60 mL/min/1.73 m2, 90 mL/min/1.73 m2, Never (we always report an exact number), Other: please specify
	10	Please indicate the 1 identifier you use for your laboratory when submitting your CMS-116 form (CLIA Application for Certification)	See list of 26 laboratory types at: www.cms.hhs.gov/cmsforms/downloads/cms116.pdf
	11	For paper-based respondents: Enter the 2-letter state or territory abbreviation where your laboratory is located	Fill in the blank

Abbreviations: NKDEP, National Kidney Disease Education Program; eGFR, estimated glomerular filtration rate; MDRD, Modification of Diet in Renal Disease; CMS, Centers for Medicare & Medicaid Services; CLIA, Clinical Laboratory Improvement Amendments.

The initial mailing and postcard yielded a response rate of approximately 30.0%. A telephone reminder call was fielded to all nonresponders who had telephone numbers recorded in the sample database. During that telephone contact, if possible, survey data were directly collected by a trained interviewer to maximize response rates. This reminder/computer-assisted telephone interview data collection effort was fielded between January 16 and February 13, 2007. A total of 4,013 laboratories responded across all 3 modes (52.7% telephone, 41.2% mail, and 6.1% web). The overall survey response rate was 63.4% (range, 58.2% to 78.6% across laboratory types). Data from all modes were merged and prepared for analysis. The number of laboratories excluded from analyses because of missing data is noted in the relevant data tables.

Analysis 

All analyses were conducted using SAS (version 9.1; SAS Institute, Cary, NC) PROC SURVEYFREQ. This procedure uses sampling weights and the finite population correction (FPC) to estimate the overall percentages, pooled variances, and 95% confidence intervals. The FPC was used in estimating the variance within strata, and the Rao-Scott χ2 test11 was used to test for differences among the strata. P values for comparisons among the first 4 strata were adjusted by using the stepdown Bonferroni method (SAS PROC MULTTEST).12 Note that 22 of 28 possible responded in the public health/insurance/health fair stratum. Although a small number, confidence intervals are still shown for this group and are expectedly wide for all items. Serum creatinine volume quartiles were developed for each stratum to test the hypothesis that higher volume laboratories are more likely than lower volume laboratories to report eGFR. This study was implemented after receiving clearance from the US Office of Management and Budget (Office of Management and Budget no. 0925-0570).