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1. Summary

  • laboratory tests are indicated on the basis of clinical examination of the patient and suspicion of the disease; in asymptomatic patients only for screening purposes
  • most errors that harm the patient are caused by the referring physician not thinking of a beneficial laboratory marker and not indicating it; numerically, most errors are found in the pre-analytical phase (patient preparation, sample collection and transport)
  • The reference range is determined by the biological variability of the marker: it includes 95% of the values of the results in the reference population, with the lowest 2.5% and highest 2.5% of the reference population always outside the reference range.
  • The decision limit is an arbitrarily determined value of a laboratory test when deciding on the presence of disease or therapeutic intervention. Diagnostic sensitivity and specificity characterize the ability of the test to separate healthy and diseased populations. Negative and positive predictive values determine the probability that a patient does or does not have the disease when the test is negative or positive, respectively.
  • each laboratory result is surrounded by uncertainty and is perceived as an interval of possible values rather than a single point
  • Disorders of natremia are associated with water metabolism; measurement of serum and urine osmolality and urine sodium is useful to determine the cause of hyponatremia and hypernatremia
  • Disorders of kalemia are most often caused by increased loss or retention by the kidneys, or a shift between ICT and ECT (hypokalemia in alkalemia, hyperkalemia in acidemia)
  • Disorders of acid-base balance are caused by causes leading to hypoventilation or hyperventilation (respiratory disorders; changes in pCO2) and disorders leading to production or loss of acids or bases (metabolic disorders; changes in HCO3, Cl-, and unmeasured anions).
  • The best clinically available marker of kidney function is the estimation of glomerular filtration rate; the best clinically available marker of kidney damage is the ratio of albumin to creatinine in the urine.The chemical examination of urine by urine dipstick is one of the most indicative and very useful tests in suspected kidney disease.
  • hepatocyte damage is detected by ALT values; cholestasis by ALP and liver failure by measuring prothrombin time
  • in diagnosis of acute myocardial infarction the most useful of laboratory tests is cardiac troponin and the change in its concentration over time; in diagnosis heart failure we use measurement of natriuretic peptides
  • CRP and procalcitonin help us to diagnose inflammatory conditions and decide on antibiotic treatment
  • osmolal gap is used in acute poisoning with osmotically active substances (ethanol, methanol, ethylene glycol), in case of suspected intoxication with frequently abused substances (drugs) we indicate orientation toxicological examination of urine
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