Contributors: Kathleen P Freeman
Species: Canine | Classification: Lab Tests
Measuring serum or plasma levels of nutrients, metabolites, enzymes, etc can provide general or specific data about organ function and disease processes. Print-off the owner factsheet on 'Samples - how they help your vet' to give to your client.
- Parameters measured include:
- Total protein Blood biochemistry: total protein.
- Albumin Blood biochemistry: albumin.
- Total globulin Blood biochemistry: total globulin.
- Gamma globulins Blood biochemistry: gamma globulin.
- Sodium Blood biochemistry: sodium.
- Potassium Blood biochemistry: potassium
- Chloride Blood biochemistry: chloride.
- Bicarbonate Blood biochemistry: bicarbonate.
- Calcium Blood biochemistry: total calcium.
- Phosphate Blood biochemistry: phosphate.
- Copper Blood biochemistry: copper.
- Iron Blood biochemistry: iron.
- Urea Blood biochemistry: urea.
- Creatinine Blood biochemistry: creatinine.
- Ammonia Blood biochemistry: ammonia.
- Glucose Blood biochemistry: glucose.
- Fructosamine Blood biochemistry: fructosamine.
- Bile acids Blood biochemistry: bile acids.
- Bilirubin - direct Blood biochemistry: direct bilirubin and total Blood biochemistry: total bilirubin.
- Cholesterol Blood biochemistry: cholesterol.
- Triglycerides Blood biochemistry: triglycerides.
- Free fatty acids.
- Creatinine kinase Blood biochemistry: creatine phosphokinase.
- Aspartate aminotransferase (ALT) Blood biochemistry: alanine aminotransferase (SGPT, ALT).
- Alanine aminotransferase (AST) Blood biochemistry: aspartate aminotransferase (AST).
- Gamma glutamyltransferase (GGT) Blood biochemistry: gamma glutamyltransferase.
- Alkaline phosphatase (ALP) Blood biochemistry: alkaline phosphatase (ALP).
- Amylase Blood biochemistry: amylase.
- Lipase Blood biochemistry: lipase.
- Trypsin-like immunoreactivity (TLI) Blood biochemistry: trypsin-like immunoreactivity.
- Hormones: cortisol Blood biochemistry: cortisol , thyroxine Thyroxine assay (free) ; parathyroid hormone PTH assay.
- Vitamin B12 Blood biochemistry: vitamin B12.
- Folate Blood biochemistry: folate.
- Para-aminobenzoic acid Blood biochemistry: para-aminobenzoic acid.
- C reactive protein.
- Biochemical tests are useful only when combined with clinical data from history and physical examination of the patient.
Source of Test Material
- Standard venipuncture Jugular venipuncture.
Quantity of Test Material
- 2 ml serum or plasma. Check with laboratory regarding minimum recommended volume for single test or in combination with other tests.
- Depends on number of laboratory tests requested.
Sample Collection Technique
- Clotted or heparinized samples spun to separate serum/plasma.
- For sample tube preference see Blood sample tube requirements table 01.
- Hemolysis, lipemia and/or and aging affects the accuracy of many tests.
Timing of test
- Unless specifically indicated, animals should be fasted overnight before sampling to avoid lipemia - this contributes to inaccurate results.
Lipemia may make sample useless for some biochemical tests.
- Refrigerate before dispatch.
- Package according to mailing regulations Transportation of diagnostic specimen.
- Avoid excessive temperatures (ideally refrigerate +4 degrees C).
- Separate serum/plasm before despatch.
- A number of different methodologies may be available for any given test.
- Many tests are routinely available - if in doubt, contact the laboratory to confirm availability of a specific test.
- The ability of a test to produce a positive result in all animals that do have disease.
A high sensitivity may be associated with a reduction in speicificity as false positive reactions may occur.
- The ability of a test to produce a positive result only in animals that do have disease.
A high specificity may be associated with a lower sensitivity as some false negative may occur.
- Predictive value = how well the test performs in a given population of animals.
- Influenced by the prevalence of the disease in the population being tested.
- Positive predictive value is probability of an abnormal test result indicating presence of disease.
- Negative predictive value is probability of a test result that is within normal limits corresponding to the absence of disease.
- Predictive value of positive test = p x sensitivity/p x sensitivity + (1-p) x (1-specificity).
- Predictive value of a negative test = (1-p) x specificity/(1-p) x specificity + P X (1-sensitivity).
- p = prevalence of disease.
- In a study of dogs with and without x-disease (diagnosed at necropsy) the following results are obtained:
- Positive with test: x-disease present 235 (TP), x-disease absent 16 (FP) - total 251.
- Negative with test: x-disease present 15 (FN), x-disease absent 200 (TN) - total 215.
- Total: x-disease present 250, total x-disease absent - 216.
- 1. Sensitivity = TPx100/TP+FN = 235x100/235+15 = 235/250x100 = 94%.
- 2. Specificity = TN/TN+FPx100 = 200/200+16x100 = 200/216x100 = 93%.
- It is estimated that x-disease occurs in about 20% of the population that will be tested.
- 3.Predictive value of a positive test = p(sens)/p(sens) +(1-p)(1-spec) = 0.20(0.94)/20(94)+(0.80)(0.07) = 0.1880/0.1880+0.0560 = 0.1880/0.2440 = 0.77. So you would expect disease in 77 out of every 100 with a positive test.
- If prevalence is only 2%: PVPT = 0.02(0.94)/0.02(0.94)+(0.80)(0.07) = 0.0188/0.0188+0.0560 = 0.0188/0.0748 = 0.25. So you would expect to have x-disease in only 25 animals out of every 100 with a positive test.
- 4. Predictive value of a negative test (PVNT) = (1-p)(spec)/(1-p)(spec)+p(1-sens).
- If prevalence is 20%: (0.80)(0.93)/(0.80)(0.93)+0.20(0.06) = 0.7444/0.744+0.012 = 0.744/0.756 = 0.98. So would expect 98 out of every 100 animals with a negative test to be truly free of x-disease.
- If prevalence is 2%: (0.98)(0.93)/(0.98)(0.93)+(0.02)(0.06) = 0.9114/0.9114+0.0012 = 0.9114/0.9126 = 0.998. So would expect 99.8 out of every 100 dogs with a negative test to be truly free of x-disease.
Technique (Intrinsic) Limitations
- In general, results of any given test are most significant when interpreted in conjunction with other laboratory and clinical findings.
Normal (Reference) Values
- Reference intervals are usually established based on statistical determination of values which includes 95% of the population of normal animals.
- This means that 5% of normal animals have a value outside this range.
- The more tests that are performed the greater the chance of one of these being outside the normal range in a healthy animal.
Do not base a clinical diagnosis on one test result.
- Reference intervals are dependent on test method used.
- Individual laboratories should supply reference intervals for their tests.
- Information about numbers and types of animals used for determination of reference intervals and statistical methods used should be available by request.
Errors and Artifacts
- Poor handling of sample prior to testing particularly:
- Lipemia (inadequate fasting).
- Hemolysis (traumatic sampling or prolonged storage).
- Test inaccurately performed (unlikely in commercial laboratory, more common in practice laboratory).
- Sample collected into inappropriate transport medium or anticoagulant (see sample tube prefence table).
- Over interpretation of results.
- Artefactual hemolysis may be caused by:
- Sampling with wet needle or syringe.
- Traumatic sample technique, ie excessive suction applied to syringe with small needle.
- Trauma with shaking sample to mix anticoagulant.
- Prolonged storage before separation.
- Centrifugation in an unbalanced centrifuge.