Interference Indices

 
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The samples indexes of lipemia (or turbidity), hemolysis and icterus are part of our chemistry results. They are provided automatically with our analyzer with any chemistry test and are very useful.  These indexes are more objective and consistent than visual assessment of a sample.  Hemolysis, lipemia and icterus in a sample can, in some cases, interfere with accurate measurement of clinical pathologic analytes through various mechanisms (for more, see interferences under test basics).

Guides as to which tests are affected at different levels for each index are provided with the chemistry results from Cornell University. These are guidelines only (many are based on data from humans and may not be applicable to animals) and should be used judiciously to assess the impact of these interferences (lipemia, hemolysis, icterus) on the chemistry results. When doing this, it is very important to interpret these values along with the clinical signs displayed by the patient and the remaining laboratory results, i.e. they should never be used alone. Remember that the effect of these interferences on chemistry results are also dependent on the analyzer and methods used. They cannot be transferred from one laboratory’s results to another. These guidelines are given below for each interference.

Lipemic index

The number reported under Lipemia on our chemistry reports is actually a measure of the turbidity of the sample. Index values are obtained by adding increasing amounts of an emulsion of fat to a sample (Intralipid®) and assessing the effect the emulsion has on chemistry values. In reality, this correlates weakly to the triglyceride concentration in the sample. In our experience, the units for lipemia correlate to gross estimates of lipemia/turbidity as follows:

 

Guide to Lipemia (Turbidity) Index
Lipemia index Gross appearance
<30 No turbidity
30-60 slightly turbid (hazy)
60-120 moderately turbid (milky)
>120 markedly turbid (creamy)

 

Cornell University provides guidelines on how turbidity (usually due to lipemia) affects sample results. These guidelines are derived from manafacturer’s recommendations and are based on data from spiking human samples with Intralipid®. These values may not apply to samples from animals and there may be species differences. These guidelines should always be used in conjunction with clinical signs and knowledge of the underlying disease processes as guides for interpreting laboratory data and determining the accuracy of reported laboratory results.

Effect of lipemia (turbidity) on chemistry results (guidelines)
Lipemia index Possible effect on tests
>50 FALSELY INCREASED:  bile acids
≥100 FALSELY DECREASED:  sodium, potassium, chloride (if triglyceride concentrations are high, e.g. > 600 mg/dL);
FALSELY INCREASED:  direct bilirubin
≥200 FALSELY DECREASED:  sodium, potassium, chloride (if triglyceride concentrations are high, e.g. > 600 mg/dL);
FALSELY INCREASED:  direct bilirubin, TIBC
≥500  FALSELY DECREASED:  bicarbonate, sodium, potassium, chloride (for electrolytes, if triglyceride concentrations are high, e.g. > 600 mg/dL);
FALSELY INCREASED:  direct bilirubin, magnesium, TIBC
≥1000 FALSELY DECREASED:  bicarbonate, LDH, sodium, potassium, chloride (for electrolytes, if triglyceride concentrations are high, e.g. > 600 mg/dL);
FALSELY INCREASED:  direct bilirubin, magnesium, TIBC;
VARIABLE EFFECT:  amylase

Hemolysis index

Hemolysis tubes

Different degrees of hemolyzed serum

The number reported under Hemolysis on our chemistry panels is a semi-quantitative measurement of the concentration of free hemoglobin in mg/dL. The image below shows a visual representation of how these values correspond to visible hemolysis in plasma samples and the table below correlates the index value to this gross appearance.

Guide to Hemolysis Index
Hemolysis index Gross appearance
<20 No hemolysis
20-100 slightly hemolyzed (pink tinged)
100-300 moderately hemolyzed (red)
>300 markedly hemolyzed (dark red)

 

Free hemoglobin in the sample has little or no effect on most assays unless the level is very high (>300 hemolytic index). Cornell University provides guidelines on how hemolysis affects sample results. These guidelines are derived from manafacturer’s recommendations and is based on data from spiking human samples with hemoglobin. These values may not apply to samples from animals and there are important species differences (for example, a high potassium is expected in hemolyzed samples from horses, camelids, pigs and sheep because potassium concentrations in the erythrocytes of these species are higher than in serum – this is not true for most breeds of dogs, other than Asian breeds, or several breeds of cattle). These guidelines should always be used in conjunction with clinical signs and knowledge of the underlying disease processes as guides for interpreting laboratory data and determining the accuracy of reported laboratory results. Species-dependent changes have been published, however these will always be method- and analyzer-dependent and are not broadly applicable to results. For more on species-dependent changes, refer to the interference section of test basics.

Effect of hemolysis on chemistry results (guidelines)
Hemolysis index Possible effect on results
≥49 This degree of hemolysis may falsely decrease the following result:  direct bilirubin (in our experience, direct bilirubin is not affected overly by hemolysis.
≥80 FALSELY DECREASED: direct bilirubin, bile acids;
FALSELY INCREASED: iron;
VARIABLE EFFECT: TIBC
≥200 FALSELY DECREASED:  bicarbonate, direct bilirubin;
FALSELY INCREASED:  AST, cholinesterase, CK, iron, LDH, potassium (species-dependent);
VARIABLE EFFECT:  TIBC
≥300 FALSELY DECREASED: bicarbonate, direct bilirubin, GGT;
FALSELY INCREASED: AST, cholinesterase, CK, iron, LDH, phosphate, potassium (species-dependent);
VARIABLE EFFECT: TIBC
≥400 FALSELY DECREASED:  amylase, bicarbonate, direct bilirubin, GGT;
FALSELY INCREASED:  AST, cholinesterase, CK, iron, LDH, phosphate, triglycerides, potassium (species-dependent);
VARIABLE EFFECT:  TIBC
≥600 FALSELY DECREASED: amylase, alkaline phosphatase, bicarbonate, direct bilirubin, GGT;
FALSELY INCREASED: AST, cholesterol, cholinesterase, CK, iron, LDH, magnesium, phosphate, total protein, triglycerides, potassium (species-dependent);
VARIABLE EFFECT:  TIBC
≥800 FALSELY DECREASED: amylase, alkaline phosphatase, bicarbonate, direct bilirubin, GGT, NEFA (by an average of 0.05 mEq/L, range of 0.03 to 0.07 mEq/L);
FALSELY INCREASED: AST, cholesterol, cholinesterase, CK, iron, LDH, magnesium, phosphate, total protein, triglycerides, potassium (species-dependent);
VARIABLE EFFECT: TIBC

Icteric index

The number reported under Icterus is an estimation of the total bilirubin concentration in mg/dL rounded to the nearest whole number.

The icterus index is not affected by lipemia and can be used to determine if there is hyperbilirubinemia, i.e. if the total bilirubin is increased, the icteric index should closely match the value seen. For example, a serum sample from a dog that has an Icterus index result of “4” has a total bilirubin concentration of approximately 3.5 to 4.4 mg/dL, which indicates that the dog has hyperbilirubinemia. However, if there is a mismatch, e.g. the total bilirubin is 2 mg/dL and the icteric index is 0, the total bilirubin result is falsely increased. This type of artifact is seen in the following situations:

  • Underfilling of heparin anticoagulant tubes: If heparin (green top) anticoagulant tubes are <3/4 full with blood, a falsely increased total bilirubin (mostly indirect bilirubin) may be seen.
  • Species that normally have lightly colored serum (e.g. birds, ruminants): Carotenoids in the diet (and serum) will contribute to the icteric index (but not the total bilirubin value).

Cornell University provides guidelines on how icterus affects sample results, e.g. icterus may falsely decrease creatinine results. These guidelines are derived from manafacturer’s recommendations and is based on data from spiking human samples with conjugated bilirubin. These values may not apply to samples from animals and there may be species differences. These guidelines should always be used in conjunction with clinical signs and knowledge of the underlying disease processes as guides for interpreting laboratory data and determining the accuracy of reported laboratory results.

Effect of icterus on chemistry results (guidelines)
Icteric index Possible effect on results
≥9 FALSELY DECREASED:  cholesterol, triglycerides, creatinine, bile acids
≥19 FALSELY DECREASED:  cholesterol, lipase, total protein, triglycerides, creatinine
≥40 FALSELY DECREASED:  cholesterol, creatinine, GGT, lipase, total protein, triglycerides, uric acid; FALSELY INCREASED:  magnesium
≥55 FALSELY DECREASED:  amylase, cholesterol, creatinine, GGT, lipase, total protein, triglycerides, uric acid; FALSELY INCREASED:  magnesium

 

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