Current Trends Laboratory Confirmation of Measles Using Capillary Blood Specimens

The Immunization Practices Advisory Committee (ACIP) has recommended that laboratory confirmation be attempted on every suspected case of measles that cannot be linked to another laboratory-confirmed case (1). To that end, a finger- or heelstick method of collecting capillary blood on filter-paper strips was evaluated for sensitivity and specificity in the laboratory diagnosis of measles.

The correlation of measles hemagglutination-inhibition (HI) antibody titers was assessed using 125 sets of capillary blood and venous serum specimens obtained from 81 individuals during investigations of sporadic and outbreak-associated cases. The close correspondence between venous and capillary HI antibody titers is indicated in Figure 1 (correlation coefficient = 0.85). Of the 125 sets, 124 (99.2%) had a less than or equal to two-fold difference in titer between the two tests. Only one of the sets (0.8%) showed a significant difference ( greater than or equal to four-fold difference in titer) between the two tests.

The ability of capillary blood specimens to confirm recent measles infection was compared with that of venous serum specimens, the standard specimen used to confirm measles.* Paired acute- and convalescent-phase venous and capillary specimens from 44 individuals were tested. These tests showed substantial agreement (Table 1). The sensitivity of tests on capillary blood was 100%, and the specificity was 96.4%.

Using staphylococcal protein A (SPA) adsorption, measles-specific IgM was assessed on six simultaneously collected venous and capillary specimens (2). IgM was detected in both venous and capillary specimens in three sets of specimens; the remaining three sets were negative for IgM in both types of specimens.

A pilot program was carried out in Georgia from February through May 1982 to determine whether a higher proportion of specimens could be obtained from suspected measles cases** if capillary-blood testing was available. During the program's initial stages, when filter-paper strips were unavailable for capillary blood testing in several counties and only venipuncture could be used, specimens were obtained from eight of 16 (50%) suspected cases including two of 10 preschool-age ( 5 years) children. In contrast, when filter-paper strips were available, capillary specimens were obtained from 36 of 37 (97%) suspected cases of measles (p = 0.0001), including all 21 of preschool age.

Personnel obtaining capillary blood specimens enthusiastically accepted this method, particularly for use among preschool-age children. All capillary specimens submitted in this pilot program were satisfactory for testing. Reported by C Baker, S Clark, D Lockridge, S Register, T Seegmuller, G Smith, C Turner, Immunization Program, JW Alley, MD, Div of Public Health, Georgia Dept of Human Resources; DG Ramras, MD, LD Fredericks, MA Harris, MK Hernandez, J Merrill, SJ Parker, CR Peter, PhD, SE Ross, MA Thompson, DrPH, WA Townsend, MD, H Tuller, PB Villalon, M Workman, MD, Dept of Health Svcs, San Diego, L Dales MD, J Chin, MD, State Epidemiologist, California Dept of Health Svcs; Viral Exanthems and Herpes Virus Br, Div of Viral Diseases, Center for Infectious Diseases, Surveillance, Investigations and Research Br, Immunization Div, Center for Prevention Svcs, CDC

Editorial Note

Editorial Note: Confirmation of indigenous measles transmission in a given area is aided by laboratory assessment of suspected cases. The eagerness of investigators to obtain specimens, the availability of personnel trained in obtaining the specimens, and the likelihood of obtaining permission to collect specimens affect the proportion of suspected measles cases that undergo laboratory testing. The data presented here indicate that a fingerstick/filter-paper method of blood collection and testing is feasible, acceptable, and accurate.

Measles HI antibody titers in capillary blood and venous sera obtained from the same individuals have been compared previously, with good agreement (3). The data presented here corroborate these findings. This method of collection and testing, if used correctly, can yield valid laboratory assessments. Filter-paper capillary blood assessments were as sensitive as venous serum assessments in detecting seroconversion and were also highly specific. IgM can probably be measured accurately using SPA adsorption.

The fingerstick method of obtaining blood is acceptable to donors, their parents, and blood collectors (4). After a brief orientation, blood collectors were able to obtain satisfactory specimens.

Any laboratory that can perform measles HI antibody tests can measure HI antibody from filter-paper strips after extracting blood from the filter papers by a standardized elution technique. Accurate testing requires a standard volume of blood, which is assured by complete filling of the areas indicated on the filter-paper strips.

The fingerstick method potentially allows all suspected cases, particularly in preschool children, access to laboratory assessment. Persons desiring information about laboratory testing for measles using capillary blood on filter-paper strips should contact the immunization programs of their state health departments.

References

1. ACIP. Measles prevention. MMWR 1982;31:217-24,229-31.

2. CDC. Serologic diagnosis of measles: MMWR 1982;31:396,401-2.

3. Brody JA, McAlister R, Haseley R, Lee P. Use of dried whole blood collected on filter paper disks in adenovirus complement fixation and measles hemagglutination inhibition tests. J Immunol 1964; 92:854-7.

4. Matthews HM. Parasitic disease: testing with filter-paper blood spots. Laboratory Management 1981;19:55-62. *Laboratory confirmation is defined as a greater than or equal to four-fold rise in measles HI antibody titer. **An illness with a generalized maculopapular rash (lasting more than a day if the report was delayed) accompanied by fever.

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