Additional information for interpreting nutrition indicators
concentrations of certain biomarkers are altered during times of infection or inflammation. For example, ferritin is a positive acute phase protein whose concentration increases during inflammation, while retinol binding protein (RBP) is a negative acute phase protein whose concentration decreases during inflammation. This means that measured concentrations may need to be adjusted for inflammation. The concentrations of acute phase protein inflammatory biomarkers, namely C reactive protein (CRP) and α-1-acid glycoprotein (AGP) indicate the presence of inflammation. The relative elevation in concentration of these biomarkers indicates the stage of the inflammatory response. For this reason, CRP and AGP are commonly measured in micronutrient surveys. More information on ththis can be found in the Bond and inflammation online tool, in Module 3: Biomarker selection and specimen handling. and at http://www.BRINDA-nutrition.org.
Fasting status and time of day at blood collection
The time since food was last consumed and the time of day that blood is collected can affect blood concentrations of key micronutrients. For example, zinc concentration is sensitive to the time of day that blood was collected, to when the last meal was ingested and to the use of hormonal birth control. The cutoff values for zinc deficiency are adjusted for the time of day (morning versus afternoon) and fasting status.
For some micronutrients, it is not certain whether the time of day or fasting status affects the interpretation of results. It is recommended to include data entry fields about the time of day, time since the last meal, physiologic (pregnant or lactating) status, use and timing of hormonal birth control, and other potentially relevant information. This allows for comparison of methods and results with subsequent surveys to see if there may be a physiologically relevant characteristic that could have influenced any difference in results.
Haemodilution, which occurs during pregnancy, alters the concentrations of circulating micronutrients. Kidney function and relative urine volume also change during pregnancy. As such, for pregnant women there are separate cutoff values for some biomarkers, in particular median urinary iodine and haemoglobin (see Module 3: Biomarker selection and specimen handling. for details). Most indicators of nutrients such as iron, vitamin A, zinc, folate and vitamin B12 do not have established cutoff values for defining deficiency during pregnancy, and few surveys are designed to have a representative sample of pregnant women. Sample size determination and data analysis for pregnant women need to be separate from those for non-pregnant WRA.
Elevation above sea level and smoking
Haemoglobin levels are affected by elevation above sea level and by smoking, thus values need to be adjusted to account for these factors prior to classifying for anaemia. Biologically implausible haemoglobin values need to be defined as adjusted (for elevation above sea level and smoking) or non-adjusted (if elevation and smoking variables are not available), then abnormal values should be flagged and potentially set to missing. This is discussed in more detail in Module 11: Data Collection Tools, Field Manual, and Database.