Submission of Laboratory Samples
Veterinary Diagnostic Services
The clinician must be aware of inherent limitations of laboratory testing in certain clinical settings. In general, veterinary diagnostic laboratories are preferred to human medical laboratories because these latter laboratories will be unfamiliar with animal diseases, normal physiology, and the responses of animals to disease.
There may also be significant differences in test methodology and normal reference ranges. These species*Contributions to previous editions by Gary P. Carlson are acknowledged. differences can cause confusion when results are evaluated on the basis of human reference intervals, which are not applicable to animals.
Various desktop or portable handheld point-of-care devices (POCD) are available to veterinarians for determination of serum chemistry, electrolytes, and acid-base balance. Point of care (POC) refers to any laboratory testing done outside the conventional laboratory and in close proximity to the patient. Many of these devices use self-contained strips, cartridges, or rotors and thus reduce errors associated with the maintenance, measurement, and mixing of reagents. Some of these devices can use heparinized whole blood or plasma. Some POCDs require refrigerated storage of reagent cartridges, which must then be warmed to room temperature before use. There are relatively few independently published data comparing the results obtained with these POCDs with standard laboratory procedures. Many of these POCDs need species-specific controls and reference intervals, as well as validation.3 Given these requirements, the American Society for Veterinary Clinical Pathology (ASVCP) has recommended the following guidelines when using them: taking a formal approach to POC testing including development of written policies regarding use, operator training, assessment of instrument analytical performance, use of validated reference intervals, and checks on accuracy of patient results reporting.4 A widely used handheld device (iSTAT) has been shown to yield comparable results to standard laboratory techniques when blood electrolyte concentrations and acid-base balance are determined in dogs and horses.5 However, it was noted that the correlation between results from this device and from standard laboratory techniques was poor for sodium in the dog and for hematocrit in the horse.
A study using the same handheld device in cattle, horse, and sheep showed that, except for sodium concentration and hematocrit in horses and sheep, correlation was good or excellent for most variables reported.5 Portable POCDs can provide rapid, accurate, and relatively inexpensive results. As technology continues to improve, wider and more general application of these devices is taking place in many large animal practice settings.Selection of Procedures
Selection of specific laboratory tests fosters logically integrated thinking and concentrates on evaluation of the primary medical problems. However, the sophisticated autoanalyzers used by large commercial laboratories can perform a battery of tests quickly and efficiently with little additional cost. These panels may be broadly defined (e.g., a general large animal health panel) or may offer a more focused evaluation of a specific organ system (e.g., liver, kidney, or muscle). The clinician must ensure that the panel selected contains all the appropriate tests for a thorough evaluation of the individual patient's medical problems identified.
The following recommendations for diagnostic panels are intended to provide a clear indication of organ damage and/ or dysfunction. The most directly applicable diagnostic procedures are listed under “Recommended,” and additional procedures that may be of benefit in certain circumstances are listed under “Optional.”
GENERAL PANEL. The broad-based general chemistry panel should provide a balanced evaluation of the most likely medical problems, and reports should include reference intervals for each analyte and for each species from which the sample was obtained.
Recommended
Glucose
Serum/plasma urea Creatinine
Creatine kinase (CK) Aspartate aminotransferase (AST) Sorbitol dehydrogenase (SDH) Y-Glutamyltransferase (GGT) Glutamate dehydrogenase (GLDH) Alkaline phosphatase (ALP) Bilirubin (direct, indirect, and total) Total protein
Albumin
(Globulin) (calculated: total protein concentration-albumin concentration)
Albumin/globulin ratio (A/G ratio) Haptoglobin (mostly food animals) Serum amyloid A (equine)
Bicarbonate or total carbon dioxide (TCO2) (often routinely reported) Sodium
Potassium
Chloride
Calcium
Phosphate Magnesium (total Mg)
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Optional
Fibrinogen
Venous or arterial blood gases
Ionized calcium
Ionized magnesium
MUSCLE PANEL.
The muscle panel should detect active skeletal and cardiac muscle injury (rhabdomyolysis) and the degree of secondary renal damage. The possible causative factors are evaluated as optional procedures, depending on the history, clinical findings, or special circumstances. Muscle biopsy with special staining may be critical to the diagnosis of specific muscle diseases such as polysaccharide storage myopathies, immune-mediated myopathy, and mitochondrial myopathy. Special tests for muscle function and the genetic basis for some myopathies are discussed in the chapters on muscle and genetic tests (see Chapters 42 and Chapter 52).Recommended
CK
AST
Muscle biopsy
Urinalysis
Blood urea nitrogen and creatinine
Optional
Alanine aminotransferase (ALT)
Myoglobin
Blood selenium/Vitamin E
Venous blood gases including L-lactate
Deoxyribonucleic acid analysis (hyperkalemic periodic paralysis [HYPP], polysaccharide storage myopathy type 1; type 2 offered but not currently recommended)
Calcium (ionized and total)
Magnesium (ionized and total)
Fractional excretion of:
Sodium
Potassium
Chloride
Myoglobin
Calcium
Cardiac troponin I (evaluation of cardiac muscle)
LIVER DISEASE PANEL. The liver panel should detect active damage to the hepatic parenchyma, involvement of the biliary system, and alteration in hepatic function.
Recommended
SDH
GDH
AST
GGT
ALP
Blood urea nitrogen (BUN)
Serum bile acids (good diagnostic utility in Equidae; insensitive in ruminants)
Blood glucose Fibrinogen Total protein Albumin
Globulin
Bilirubin (direct also called conjugated, indirect also called free or unconjugated, and total bilirubin)
Complete urinalysis
Optional
Liver ultrasound
Liver biopsy/culture
Blood ammonia (arterial preferable to venous samples and always run with a control animal sample)
Coagulation panel
Bromsulphalein clearance halftime test (BSP t/1/2)
RENAL DISEASE PANEL.
The kidney panel and urinalysis provide an estimation of renal function and should indicate the location and nature of the damage to the urinary tract.Recommended
BUN (or serum/plasma urea)
Creatinine
Na
K
Cl
Calcium Phosphate Total protein Albumin Globulin Complete urinalysis
Optional
Renal ultrasound
Renal biopsy Urine culture
Urine/plasma osmolality ratio Fractional excretion of:
Sodium
Potassium
Chloride
Calcium (ruminants)
Endogenous creatinine clearance
Sodium sulfanilate clearance
GASTROINTESTINAL DISEASE PANEL. The gastrointestinal disease panel should include evaluation of acid-base status, fluid and electrolyte balance, and renal function, which are common complicating features of gastrointestinal diseases. Additional optional or special diagnostic procedures may be necessary in calves or foals with neonatal diarrhea, in horses with colic, and in ruminants with gastrointestinal stasis or displacements.
Recommended
Packed cell volume (PCV) Total protein (TP)
Sodium Potassium Chloride Calcium (ionized and total) Venous blood gases, pH, bicarbonate Strong ion difference (SID) calculation Anion gap
Strong ion gap calculation BUN (serum/plasma urea) Creatinine
Glucose
Peritoneal fluid cytology Peritoneal fluid protein Fecal occult blood Fecal parasites Ruminal fluid pH and analysis Rumen fluid chloride concentration
Optional
Peritoneal fluid (glucose, L-lactate, Serum Amyloid A (SAA), and pH)
Serum immunoglobulin
Plasma lactate (D and L forms) Fecal protozoa
Fecal culture
Fecal Clostridium difficile toxin testing
Fecal Clostridium perfringens Enterotoxin A Fecal cytology
Rectal biopsy (histology, culture) Absorption tests:
Glucose
D-Xylose
Oral lactose tolerance test
Metabolic Profiling
The health status and productivity of dairy cattle, swine, and other food animals maintained in large confined groups involve a delicate balance among metabolic events, nutrition, agents of disease, management, and environmental factors.
In these production units, the health status of the herd as a whole is of paramount importance. Subclinical disease and physiologic or nutritional imbalances may contribute to suboptimal productivity. Most productivity problems in these settings are multifactorial.6 Defining and finding solutions to these problems can be a difficult and complicated task. Sequential assessment of weight gains, body condition scores, milk quality (fat and protein concentrations), and milk production are useful measures of the presence of subclinical production disorders but do not identify the cause. Metabolic profiling is a tool that has been used with mixed results. Blood samples are drawn from a number of randomized individuals as representative of the group as a whole. Some have recommended the submission of pooled serum samples from representative individuals, much like the use of bulk tank tests as a reflection of the general level of mastitis in a herd; however, sampling may also be done routinely and sequentially (longitudinally). In dairy cattle this may be done during gestation or lactation but frequently focuses on the periparturient period when a combination of nutritional and metabolic events often contributes to costly production disorders. Metabolic profiles might include most of the parameters listed under the recommended general panel with the addition of magnesium, total cholesterol, nonesterified fatty acids (NEFA), and β-hydroxybutyrate (BHB). BHB, NEFA, and cholesterol may provide an indication of energy balance, whereas serum/plasma urea, creatinine, total protein, albumin, globulin, and CK may be helpful in assessing protein status along with urea levels in milk. In certain settings, trace minerals or fat-soluble vitamin concentrations may be important indicators of underlying nutritional problems. Milk citrate measurement is a promising biomarker of physiologic imbalance in postpartum cows on farm.7 To monitor specific subclinical entities such as subacute ruminal acidosis (SARA), subclinical hypocalcemia and subclinical ketosis, measurements of rumen pH, urine pH, and β-hydroxybutyrate in blood for each disease, respectively, have been used.8 It has been suggested that the interpretation of the herd test results should depend on the biology of the specific metabolic disease. When assessing a metabolite that is associated with disease depending if it is above or below a cut point, this test should be interpreted as a proportional outcome above the cutoff point.9 Metabolic profiling is not a substitute for careful clinical examination, analysis of husbandry practices, and ration analysis, but it may play a useful role in some modern large-scale operations in which a variety of subclinical, mostly management-related problems may quickly translate into financial disaster.