Pre analytical Considerations

• Physiological variables that influence laboratory test results


  • Altitude
  • Dehydration
  • Diet
  • Diurnal/Circadian Variations
  • Drug Therapy
  • Exercise
  • Fever
  • Gender
  • Intramuscular Injection
  • Jaundice
  • Position
  • Pregnancy
  • Smoking
  • Stress
  • Temperature and Humidity
• Reference ranges/intervals


  Values expected of healthy individuals used to evaluate test results
• Basal state


  Resting metabolic state of the body early in the morning after fasting for approximately 12 hours

  
• Basal state


  • Ideal for establishing reference ranges on inpatients because the effects of diet, exercise, and other controllable factors on test results are minimized or eliminated
  • Influenced by physiologic patient variables such as age, gender, and conditions of the body that cannot be eliminated
• Age


  Values for some blood components vary considerably depending upon the age of the patient
• Altitude


  Test results for some blood analytes show significant variation at higher elevations compared with results at sea level

  
• Altitude


  • Red blood cell (RBC) counts are higher at higher elevations
  • C-reactive protein and uric acid increase at higher elevations
  • Urinary creatinine and plasma renin decrease at higher elevations
• Dehydration


  Decrease in total body fluid that causes hemoconcentration, a condition in which blood components become concentrated in the smaller plasma volume
• Dehydration


  • Affects RBCs, enzymes, iron, calcium, sodium, and coagulation factors
  • Makes it difficult to obtain blood specimens
• Diet


  Blood analyte composition can be altered by the ingestion of food and drink
• Diet


  • Fasting eliminates most dietary influences on testing
  • Patients typically fast 8-12 hours before specimen collection
• How diet affects analytes

  • Ammonia, urea, and uric acid levels elevated with high-protein diets
  • Cortisol and ACTH levels increased with caffeine consumption
  • Glucose levels increased with carbohydrate/sugar consumption
  • Hemoglobin levels decreased and electrolyte balance altered by excessive fluid intake
  • Lipid levels increased with fatty food consumption
• Diurnal/Circadian variations


  Levels of many blood components normally exhibit daily or 24-hour cycle fluctuations
• Diurnal/Circadian variations


  • Affected by factors like posture, activity, eating, light/dark, and sleep/wake cycles
  • Tests influenced by diurnal variation are often ordered as timed tests to be collected close to the ordered time

  
• Drug therapy


  Some drugs alter physiological functions, causing changes in the concentrations of certain blood analytes
• How drugs affect analytes


  • Chemotherapy drugs decrease blood cells
  • Drugs toxic to liver increase liver enzymes
  • Opiates increase liver and pancreatic enzymes
  • Oral contraceptives affect ESR and vitamin B12
  • Steroids and diuretics cause pancreatitis and increase amylase and lipase
  • Thiazide diuretics affect electrolytes and nitrogenous waste
• Drugs


  Can cause a decrease in blood cells, especially WBCs and platelets
• Drugs toxic to the liver


  Evidenced by increased levels of liver enzymes such as aspartate aminotransaminase (AST), alkaline phosphatase (ALP), and lactate dehydrogenase (LDH) and decreased production of clotting factors
• Opiates


  Such as morphine increase levels of liver and pancreatic enzymes
• Oral contraceptives


  Can affect the results of many tests, e.g. elevate the erythrocyte sedimentation rate (ESR) and decrease levels of vitamin B12
• Steroids and diuretics


  Can cause pancreatitis and an increase in amylase and lipase values
• Thiazide diuretics


  Can elevate calcium and glucose levels and decrease sodium and potassium levels. Can also increase blood levels of nitrogenous waste such as urea (azotemia)
• Drugs can interfere with the actual test procedure, causing false increases or decreases in test results
• A drug may compete with the test reagents for the substance being tested, causing a falsely low or false-negative result, or the drug may enhance the reaction, causing a falsely high or false-positive result
• It is ultimately up to the physician to prevent or recognize and eliminate drug interferences, which can be a complicated issue that requires cooperation between the physician, pharmacy, and laboratory
• CRUD


  Acronym for "compounds reacting unfortunately as the desired" - substances that interfere in the testing process
• Phlebotomists can note on the requisition when they observe medication being administered just prior to blood collection
• CAP guidelines


  Drugs that interfere with blood tests should be stopped or avoided 4 to 24 hours prior to obtaining the blood sample for testing. Drugs that interfere with urine tests should be avoided for 48 to 72 hours prior to the urine sample collection
• Exercise


  • Affects a number of blood components, raising levels of some and lowering levels of others
  • Effects vary depending on the patient's physical condition and the duration and intensity of the activity
  • Moderate to strenuous exercise appears to have the greatest effect
  • Levels typically return to normal soon after the activity is stopped
• Effects of exercise on blood components


  • Reduced arterial pH and PaCO2 levels
  • Elevated glucose, creatinine, insulin, lactic acid, total protein, and potassium
  • Increased skeletal muscle enzyme levels like creatine kinase (CK) and lactate dehydrogenase (LDH)
  • Increased cholesterol levels (up to 6% for up to an hour after vigorous exercise)
  • Affects hemostasis, e.g. increased platelet clumps
• Athletes generally have higher resting levels of skeletal muscle enzymes, and exercise produces less of an increase
• Fever


  Affects the levels of a number of hormones, e.g. increases insulin and cortisol levels, disrupts diurnal variation of cortisol
• Gender


  • Affects the concentration of a number of blood components, with separate normal values for males and females (e.g. higher RBC, Hgb, Hct for males)
• Intramuscular injection


  Can increase levels of creatine kinase (CK) and skeletal muscle LDH, so these should be drawn before injection or at least 1 hour after
• Jaundice/Icterus


  Condition characterized by increased bilirubin in the blood, leading to yellow discoloration. Can interfere with chemistry tests based on color reactions
• Jaundice in a patient may indicate liver inflammation caused by hepatitis B or C virus
• Body position


  • Going from supine to upright can decrease plasma volume up to 10%, increasing concentration of protein-bound components
  • Significant increase in potassium within 30 minutes of standing
  • Up to 15% variation in total and HDL cholesterol with position change
  • Plasma aldosterone and renin can double within an hour with position change
• The National Cholesterol Education Program recommends that lipid profiles be collected in a consistent manner after the patient has been either lying down or sitting quietly for a minimum of 5 minutes
• Calling outpatients into the drawing area and having them sit in the drawing chair while paperwork is readied can help minimize effects of postural changes on some analytes
• Pregnancy


  Causes physiologic changes that require comparing test results to reference ranges for pregnant populations, e.g. dilution effect lowering RBC counts