MUN QUALITY CONTROL

 

How are the laboratories compared?

Each month Eastern Laboratory Services selects 12 bulk tanks of milk from commercial dairy farms.  Tanks are selected to have a wide range of composition for urea, fat, protein, and somatic cell counts.  Each tank is thoroughly mixed and samples are drawn for use in Quality control testing.  Sets of 24 samples (2 samples from each of the 12 bulk tanks) are sent to each DHIA laboratory and to the reference testing laboratories.  The results of the MUN testing of these samples are sent to the National DHIA office where the data is assembled and sent to our laboratory QC technical specialist Paul Sauve for review and analysis.

 

 

Urea in milk

Urea is a very small but important component of milk.  Milk Urea Nitrogen (MUN) is measured in milligrams per deciliter (mg/dl) ie grams per 100 liters.  100 liters of typical milk contains 3600 grams of fat,  3200 grams of protein, but only 12 grams of urea.  This low concentration makes accurate testing for differences in urea much more difficult than testing for differences in fat or protein.

 

 

Types of equipment used in DHIA milk labs

Two types of tests (wet chemistry and Infrared) are performed in DHIA milk laboratories.

 

Wet Chemistry tests are performed using Chemspec, Eurochem, Skalar, and Fiastar instruments.  These tests require running the samples through a different process than the testing for fat and protein.

 

InfraRed (IR) testing using Foss instruments predicts the concentration of urea from the spectrum of light reflected off chemical bonds in the milk from an infrared light source.  This is the same method and instrument used to measure fat and protein composition in milk testing laboratories, and the MUN test is performed as the sample is being tested for fat and protein. 

 

 

Measuring the true MUN in the sample

The international standard method for testing Urea in milk is the differential Ph method.  Variations of this method are used in several instruments, but the Eurochem CL10 is recognized by the International Dairy Federation (IDF) as the standard method for reference testing of urea in milk..  The National DHIA Quality Control samples are sent to six laboratories where they are tested with Eurochem CL10 instruments, and the average result for each sample is used as the true MUN for that sample.

 

 

Parameters used to measure accuracy of testing

Mean Difference (MD). 

MD measures the amount (mg/dl) that the laboratory results are higher(+) or lower (-) than the true MUN when results are averaged for all 24 samples. 

 

Standard Deviation of Difference (SDD).

SDD measures the sample to sample variation in differences between the lab result and true MUN.  SDD measures the average absolute value of individual sample differences.  If SDD is large and MD is small the + and – sample differences are offsetting each other and measurement errors are large but random.

 

Slope of Regression. 

Slope measures the increase in MUN measured by the instrument for each unit of increase in true MUN in the samples, and is expected to be 1.00.  Some refer to the slope as the recovery of urea by the instrument.  The slope is calculated as the coefficient of regression of the instrument MUN on true MUN for the 24 samples tested.

 

Correlation

Correlation measures the agreement between the instrument reading and true MUN for the 24 samples with respect to ranking the samples on MUN content.  It is calculated as the coefficient of correlation and measures the relative differences as well as ranking.  An instrument can have a high correlation even though it is consistently overestimating or underestimating the MUN content (MD different from 0.00) of all the samples.

 

A table summarizes the performance of participating laboratories for each of the 3 most recent months.  Some laboratories have more than one line (instrument) and several use more than one type of instrument.  Laboratories using Infrared instruments generally recommend that the whole herd be tested for MUN regularly and that averages of cows in the same group and stage of lactation be used to monitor herd nutrition.  The wet chemistry methods are more accurate and are recommended when individual sample accuracy is required, such as individual cow evaluation or bulk tank sample analysis.

MUN QUALITY CONTROL

 

How are the laboratories compared?

Each month Eastern Laboratory Services selects 12 bulk tanks of milk from commercial dairy farms.Tanks are selected to have a wide range of composition for urea, fat, protein, and somatic cell counts.Each tank is thoroughly mixed and samples are drawn for use in Quality control testing.Sets of 24 samples (2 samples from each of the 12 bulk tanks) are sent to each DHIA laboratory and to the reference testing laboratories.The results of the MUN testing of these samples are sent to the National DHIA office where the data is assembled and sent to our laboratory QC technical specialist Paul Sauve for review and analysis.

 

 

Urea in milk

Urea is a very small but important component of milk.Milk Urea Nitrogen (MUN) is measured in milligrams per deciliter (mg/dl) ie grams per 100 liters.100 liters of typical milk contains 3600 grams of fat,3200 grams of protein, but only 12 grams of urea.This low concentration makes accurate testing for differences in urea much more difficult than testing for differences in fat or protein.

 

 

Types of equipment used in DHIA milk labs

Two types of tests (wet chemistry and Infrared) are performed in DHIA milk laboratories.

 

Wet Chemistry tests are performed using Chemspec, Eurochem, Skalar, and Fiastar instruments.These tests require running the samples through a different process than the testing for fat and protein.

 

InfraRed (IR) testing using Foss instruments predicts the concentration of urea from the spectrum of light reflected off chemical bonds in the milk from an infrared light source.This is the same method and instrument used to measure fat and protein composition in milk testing laboratories, and the MUN test is performed as the sample is being tested for fat and protein.

 

 

Measuring the true MUN in the sample

The international standard method for testing Urea in milk is the differential Ph method.Variations of this method are used in several instruments, but the Eurochem CL10 is recognized by the International Dairy Federation (IDF) as the standard method for reference testing of urea in milk..The National DHIA Quality Control samples are sent to two laboratories where they are tested with Eurochem CL10 instruments, and the average result for each sample is used as the true MUN for that sample.

 

 

Parameters used to measure accuracy of testing

Mean Difference (MD).

MD measures the amount (mg/dl) that the laboratory results are higher(+) or lower (-) than the true MUN when results are averaged for all 24 samples.

 

Standard Deviation of Difference (SDD).

SDD measures the sample to sample variation in differences between the lab result and true MUN.SDD measures the average absolute value of individual sample differences.If SDD is large and MD is small the + and – sample differences are offsetting each other and measurement errors are large but random.

 

Slope of Regression.

Slope measures the increase in MUN measured by the instrument for each unit of increase in true MUN in the samples, and is expected to be 1.00.Some refer to the slope as the recovery of urea by the instrument.The slope is calculated as the coefficient of regression of the instrument MUN on true MUN for the 24 samples tested.

 

Correlation

Correlation measures the agreement between the instrument reading and true MUN for the 24 samples with respect to ranking the samples on MUN content.It is calculated as the coefficient of correlation and measures the relative differences as well as ranking.An instrument can have a high correlation even though it is consistently overestimating or underestimating the MUN content (MD different from 0.00) of all the samples.

 

A table summarizes the performance of participating laboratories for each of the 3 most recent months.Some laboratories have more than one line (instrument) and several use more than one type of instrument.Laboratories using Infrared instruments generally recommend that the whole herd be tested for MUN regularly and that averages of cows in the same group and stage of lactation be used to monitor herd nutrition.The wet chemistry methods are more accurate and are recommended when individual sample accuracy is required, such as individual cow evaluation or bulk tank sample analysis.