Tuesday, August 5, 2014

Preanalytical Variables : Controllable or uncontrollable (Part 3)

Standardization of specimen collection practices help reduce errors due to preanalytical variables.
  • Major physiological variables include posture, prolonged bed rest, exercise, physical training,
  • circadian variation and travel.
  • Other physiological variables include diet, lifestyle, stimulants, drugs, herbal preparations,
  • recreational drug ingestions etc.
Controllable Parameters
  • Posture – Changes water and plasma protein distribution. Many hormones’ values are
  • affected, some of them drastically. Related to that, electrolytes’ values also change.
  • Prolonged bed rest – Fluid retention occurs and related changes occur. Ca, Na, K, PO4 , SO4 changes. Also enzymes and hormones.
  • Exercise – Static exercise – ATP, hormones etc. due to fluid shift.
  • Moderate exercise – Increases glucose levels, lactate. Decreases pH and pCO2. Increased
  • enzymes, AST, LDH, aldolase etc. Reduces cholesterol levels.
  • Strenuous exercise – Hypoglycemia, increased glucose tolerance. Increased lactate, plasma proteins, CK levels.
  • Renin and cortisol increased dramatically.
  • Physical training – Higher skeletal muscle enzymes. Urea, urate, creatinine, thyroxine higher. Lipids dramatically reduced.
  • Circadian rhythm – Serum iron (50%), cortisol, potassium, most hormones except FSH and LH. Other physiological factors can affect circadian rhythm.
  • Travel – Affects pituitary and adrenal function. Related hormones show variation.
  • Diet – Changes occur within a few days of changed diet.
  • High protein diet increases urea, cholesterol, ammonia and urate.
  • High fat diet – Produces negative nitrogen balance and increases cholesterol.
  • High starch diet – Reduces cholesterol levels. Many small meals rather than one heavy meal has a beneficial effect on cholesterol profile.
  • Food ingestion – Overnight affects many variables.
  • Meals can increase cholesterol, urea nitrogen etc. for up to 12 hours Glucagon, insulin stimulated by protein rich meal.
  • Alkaline tide occurs post-prandially.
  • Glucose is the analyte maximally altered by food intake.
  • Vegetarianism – Reduced lipids, reduced B12.
  • Malnutrition – Proteins reduced. Lipid less than desired. Usually, glucose levels are maintained. Cortisol increased, thyroid hormones reduced.
  • Fasting and starvation – Used to treat obesity. Starvation ketosis may occur. Hormone profile altered. Transient increase in body water.
  • Lifestyle (smoking and alcohol) – Changes in many enzymes and hormones which affect laboratory result adversely.
  • Drugs – Can have in vivo and in vitro effects on laboratory results. All drugs can alter laboratory results substantially.
  • Extent varies according to specific drug.
  • Herbal preparations – Aloe vera, sandalwood and cascara segrada can cause hematuria and albuminuria on long-term use. Hypokalemia, hemolytic anemia, liver damage, microcytic anemia, hyperthyroidism, hypercoagulability and cholestatic jaundice are other side effects.
  • Amphetamines – Increase FFA.
  • Morphine – Increases amylase, lipase, ALT, AST, ALP and bilirubin. Decreases insulin, norepinephrine, neurotensin.
  • Heroin – Increases cholesterol, potassium, thyroxine. pCO2 increased, pO2 decreased. Albumin reduced.
  • Cannabis – Increases Na, K, urea, Cl, insulin. Reduces creatinine, glucose and urate.
Non-controllable Variables
  • Biological influences – Heredity, age, sex and race can affect individual lab results.
  • Age - Affects many parameters dramatically.
  • Sex – After puberty, changes in ALP, AST, ALT, CK and many other hormones are seen.
  • Race – Some parameters are affected by ethnicity of the individual.
Environmental Factors
  • Altitude – Hemoglobin, hematocrit. Plasma Na, K not affected.
  • Ambient temperature – Decreased plasma proteins and potassium. Extensive sweating can cause hemoconcentration.
  • Residence – Geographical location can change lipid profile, trace element distribution etc. Urban areas can have higher levels of pollutants in blood.
Long-term Cyclical Changes
  • Seasonal influences – Relatively small compared to a tourniquet. Calcium and vitamin D levels change with extent of sunlight received.
  • Menstrual cycle – Hormones, catecholamines, cholesterol and minerals change with menstrual cycle.
Underlying Medical Conditions

1. Obesity – Lipid profile, hormones, glucose tolerance, pyruvate, lactate etc. vary.
2. Blindness – Hypothalamic-pituitary axis stimulation is lost. Low Na, Cl, glucose seen. Impaired renal function and
negative nitrogen balance seen.
3. Stress – Many hormones and other analytes altered by stress.
4. Pregnancy – Depending on duration of pregnancy concentration of many analytes can vary.
5. Fever – Early hyperglycemia and changes in hormones. Changes in electrolyte levels.
6. Shock and trauma – Lipids, hormones can be altered by shock. Fluid balance, muscle related parameters can vary.
7. Transfusions and infusions – Proteins, water and electrolytes can vary.

(Source: Supplementary information, Textbook of Biochemistry for Medical Students, 7th Edn.)
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