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Uppsala University Department of Women’s and Children’s Health International Maternal and Child Health SE-75 185 Uppsala Sweden E-mail: joel.monarrez{at}kbh.uu.se
Dear Sir:
We read with interest the validation study for ferritin in capillary blood by Ahluwalia et al (1). As pointed out by the authors, there is a need for "tools that are applicable for field use in remote areas." Subsequent to the publication of the spot ferritin assay for samples dried on filter paper (2), we used this method to measure serum ferritin in a survey conducted in a very isolated indigenous group in northern Mexico (3) after adapting the method for use under field conditions (4). We would like to share some of the difficulties encountered when using this technique and reflect on its potential for use in remote settings.
First, although the filter-paper method avoids the need to freeze samples, samples still need to be centrifuged. In our survey, we ran a microcentrifuge from a car battery with the use of a converter to transform 12-V direct current to 110-V alternating current. Transporting and using this equipment (weighing up to 25 kg) can be difficult in isolated locations. Second, breaking the microcapillary tube to extract the serum exactly between the serum and the white cell layer can cause spillage, especially if done by a nonexperienced person. Third, extracting 20 µL serum accurately can be done only with an expensive ($200) micropipette fitted with a precision micrometer-adjustment device; otherwise, some measurement error is expected. Fourth, because of the small diameter of microcapillary tubes, the samples must be centrifuged immediately because the blood clots within minutes, potentially obstructing the extraction of serum. Using tubes with anticoagulant solves this problem, but measuring ferritin in plasma rather than in serum will result in higher within- and between-sample variation (5).
A deep prick with a long-tip lancet is needed to obtain an adequate amount of free-flowing capillary blood. After the prick, the first drop is wiped out, the second drop is used for measuring hemoglobin with a HemoCue (HemoCue, Inc, Mission Viejo, CA), and the last 3–4 drops are used to fill a 75-µL microcapillary tube. The amount of serum available diminishes as the hemoglobin concentration increases; thus, in persons with adequate iron status, a larger volume might be needed to provide the 20 µL serum required for the analysis. A finger prick is often considered less invasive and more acceptable than is venipuncture. Although this could be true when obtaining 2 drops of blood, it might not be so if 6 or more drops are taken. Many of the women we surveyed stated that finger-prick sampling was more painful than was having venous blood drawn.
In conclusion, specialized and relatively expensive equipment and well-trained personnel are needed to use the ferritin assay for capillary samples spotted on filter paper. These factors need to be taken into account when planning surveys in remote and isolated field settings.
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