Rationale

Rationale in Performing Metabolic Profiling

 

• Define the problem to be addressed.

Metabolic profiling should not be a completely random sample collection process.  One should have a plan in mind in approaching a problem. Define a given herd situation by asking a specific question.  Are the heifers experiencing subclinical ketosis?  Why are mature cows experiencing more retained placenta?  Why is the herd experiencing more periparturient disease?  Consider pertinent comparisons of interest relative to the defined problem and identify which cow populations are of concern to be sampled.

Once the herd problem has been defined a grouping strategy for sample collection can be constructed (Table 1). In addressing transition cow problems, blood analyte concentrations from cows just prior and immediately following calving are the most diagnostic. As a result of tremendous individual variation, cows should not be sampled within 3 days prior to or following calving. Others suggest samples immediately pre- and postpartum be avoided citing large analyte variability and recommend sampling fresh cows at 25-80 days in milk. Although blood analyte concentrations from far off dry cows (> 30 days prior to calving) are not predictive for postpartum disease risk, results can be used as a reference point for comparison to other groups, or values may be diagnostic within themselves for some disease entity. The group or groups of cows selected for analysis will depend upon the problem definition and desired sampling approach.

Cows to be selected within the defined groups for a metabolic profile should be free of obvious clinical disease. By selecting cows defined as “clinically normal”, outlier analyte concentrations associated with disease are removed, thus better highlighting potential differences resulting from nutritional or subclinical disease problems. One may elect to sample cows affected with specific diseases for comparison to cows of similar days in milk that are not affected. Differences in blood analyte concentrations between clinically affected and unaffected cows may provide some direction as to underlying problems associated with disease pathogenesis.

Table 1. Suggested grouping strategies for collecting blood samples in completing metabolic profile testing using individual or pooled samples.

• Define the testing procedure approach.
  

Metabolic profiling utilizes the same clinical chemistry tests performed in disease diagnosis. However, testing methods are herd-based for metabolic profiling rather than individual-based for disease diagnosis. Herd-based testing can be categorized into two approaches, targeted diagnostics and screening tool.

The screening tool approach is consistent with traditional metabolic profiling methods where multiple analytes are determined within selected group or groups of cows. Determination of multiple analytes is predicated on the concept that periparturient metabolic disease is a result of the cow’s inability to maintain coordinated interrelationships between lipid, glucose and amino acid metabolism. A screening tool approach to metabolic profiling can be used as a broad-based diagnostic evaluation of herd nutritive status, assessment of disease risk factors, or indicator of potential factors responsible for disease conditions. Limitations to the screening tool approach are high testing costs and potential interpretation issues. A pooled-sample process has been advocated to address cost concerns and maintain a wide analyte array in assessing herd nutritional or disease risk status. Predictive disease risk relationships have been well established with specific analytes, though multiple analyte indices or analyte combinations may provide a better indication of metabolic stability or instability. Unfortunately, few data are available to provide sound reference values for interpretation.

The targeted diagnostic approach utilizes well defined diagnostic analytes to determine herd risk for specific “gateway” periparturient diseases. Elevated prepartum NEFA concentration and postpartum BHB concentration are recognized risk factors for ketosis and left displaced abomasum. Low blood calcium concentration immediately postcalving is a risk indicator for subclinical hypocalcemia. Blood UN is a potential indicator for assessing herd protein status. In this approach, specific analyte concentration is determined and compared to specific threshold criteria. Percent of individuals above (NEFA and BHB) or below (calcium) is used to interpret herd disease risk. Urea nitrogen values are interpreted as a mean value for the individuals within a defined group. Individual testing, lower testing costs, and ease of interpretation are strengths of this approach. Limitation of this approach is scope of analytes determined.

Which approach to be used in evaluating a herd will depend upon the problem to be addressed, herd size, and cost limitations. Smaller dairy herds (< 120 cows) will not have a large enough population of animals to be sampled within defined physiologic groups for the screening tool approach compared to large herds. With limited animal numbers, individual testing or collecting samples over time are possible approaches. Costs are the single most limiting factor to metabolic profiling. Multiple analyte testing services range in cost from $17 to $50 per sample depending upon the number of blood analytes measured and laboratory pricing structure. This makes individual testing in multiple groups nearly cost prohibitive, thus the rationale for pooled samples. Using the single analyte approach, cost may range from $3 to $10 per sample depending upon specific analyte of interest and laboratory pricing structure.