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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. |