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Letter to the Editor |
1 Department of Nutritional Sciences Pennsylvania State University University Park, PA E-mail: nxa7{at}psu.edu
2 Department of Biochemistry and Molecular Biology University of Colombo Sri Lanka
Dear Sir:
We (1) recently developed a filter-paper method for measuring serum ferritin and validated its use in young children in a field setting in Sri Lanka (2). Monárrez-Espino et al (3) measured serum ferritin with the use of our filter paper–based dried serum spot (DSS) ferritin assay (1) and measured hemoglobin with the HemoCue method (HemoCue, Inc, Mission Viejo, CA) in young indigenous women in northern Mexico and showed that anemia was mainly due to iron deficiency (3). In their letter, Monárrez-Espino and Greiner share their experience from that survey in a "very isolated" indigenous group in northern Mexico and raise certain issues pertaining to the process of collecting blood, obtaining serum, and preparing DSS samples on filter paper. We will address the points raised in this order.
Blood sampling, whether venous or capillary, involves some degree of discomfort. Capillary blood sampling offers the advantage of being more acceptable to certain groups, such as young infants, children, and the elderly, and may be the only option available in certain cultures. Capillary blood can be collected by trained field technicians rather than by para-health professionals, who are usually required for venous blood collection. The training of field technicians to collect capillary blood samples and to separate serum is simple, but necessary, and requires a minimal amount of time (typically 1 d). In the Sri Lankan study (2), a trained laboratory technician collected the finger-prick samples into 
2 capillary tubes (75 mm each). Thus, 150 µL blood was typically collected and provided more than an adequate volume of serum necessary for the DSS preparation.
From our experience in Sri Lanka (2) and Guatemala (N Ahluwalia, J Bulux J, NW Solomons, unpublished observations, 2001) and that of others (4), whole blood ferritin is neither a reliable nor a valid tool in which to assess body iron stores, and results obtained from plasma ferritin vary (5). Thus, serum must be used for measuring ferritin to assess iron status. In our experience, when capillary tubes with an internal diameter of 1 or 1.5 mm are used for obtaining serum, the percentage of tubes that are rendered unusable is small.
Monárrez-Espino and Greiner present the difficulties experienced with the transport and use of a microcentrifuge in their survey in a "very isolated" community where electricity was not available. In our experience in Sri Lanka (2) and Guatemala (N Ahluwalia et al, unpublished observations, 2001), we did not encounter these difficulties. The samples were transported to the laboratory and centrifuged within 1–1.5 h of collection in Sri Lanka and centrifuged within 1–1.5 h of collection directly in the field with a microcentrifuge (weight: 10 kg) in Guatemala. In a highly remote setting where electricity is not available, some adaptations to centrifuging blood samples are necessary.
In our study with Sri Lankan children (2), the technician scored and broke off the spun capillary tubes without spillage after only minimal training. However, to address and potentially overcome this step—in collaboration with investigators from the Center for Studies of Sensory Impairment, Aging and Metabolism (Guatemala)—we recently finished a study in periurban Guatemala in which we used a special pipette and dispenser that make use of the blood clot itself to push a fixed volume of 20 µL serum directly from the capillary tube. The preliminary results of this study are promising (6).
Finally, in the Guatemalan study, we also evaluated other field user-friendly techniques, such as cutting out a circle with a diameter equivalent to 20 µL serum from the DSS and measuring ferritin using our spot assay (2). We found that the DSS (blot and cut) approach provided ferritin values that did not differ significantly from values obtained from serum samples stored frozen until analyzed with the traditional method (6). We plan to publish the results of this study in the coming year. The cost of the pipette (range: $65–$235) or the special dispenser ($200) is a one-time cost only and most laboratories may already have this simple equipment.
In conclusion, the spot ferritin assay is based on DSS samples that can be prepared from either venous or capillary blood on filter paper by fieldworkers with minimal, but essential, training with the use of readily available or easily obtainable equipment. The advantages of this method need to be kept in mind when considering its use. These advantages include the lack of need for a cold chain, the stability of ferritin in DSS samples stored at room temperature for up to 1 mo, the ease of shipping DSS samples, and the reduced likelihood of transmission of certain blood-borne pathogens.
REFERENCES
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