For the detection of Vitamin A and iron deficiency it is possible to measure certain proteins in blood (RBP for Vitamin A deficiency, Ferritin and sTfR for iron deficiency) with a sensitive and inexpensive Sandwich ELISA technique (Erhardt JG et. al. 2004). It can also be easily combined with the measurement of CRP and AGP as indicators for acute and chronic infection. The infectious status can be of interest by itself but also be used to correct RBP and ferritin which are influenced by infection. More information on Vit. A and Ferritin can be found in these extensive and relatively new reviews (they have a bit too much information, therefore selective reading is useful).
The combined Sandwich ELISA technique needs some experience and there are problems with the availability of the antibodies but the advantage is that the 5 proteins can be measured for 5 USD/sample (in average 1 USD per protein) with a high throughput procedure in bigger sample sizes (up to 2500/project). As material serum from venous or capillary sampling can be used and already 25 ul are sufficient to do a double measurement of these 5 proteins. Therefore a finger or heel prick is usually sufficient to get enough blood for doing these measurements.
The best material for the measurement of the proteins is serum. Heparin plasma usually forms cryoprecipitates after a freeze/thaw cycle which can block the tips of the pipettor and EDTA plasma can have problems with the stability of sTfR after freeze/thaw cycles.