• Clinical science

Laboratory medicine

Abstract

Laboratory medicine involves the analysis and evaluation of body fluids such as blood, urine, or CSF, the results of which are important for the prevention, diagnosis, and staging of diseases. Laboratory medicine plays an important role in daily clinical practice; however, the evaluation of results should always take into account the patient's medical history, as well as clinical and diagnostic findings. In addition to the basics of laboratory medicine, this learning card covers important laboratory parameters such as liver function tests and iron metabolism. Further parameters of clinical relevance may be found in other learning cards and are listed in the section “Overview of important laboratory values”. Current laboratory reference values can be found under “Tips and links” below.

Basics of laboratory analysis

Pre-analytical phase

The pre-analytical phase encompasses the selection of relevant diagnostic tests, as well as the collection and transport of samples.

Errors that strongly affect results frequently occur in the pre-analytical phase!

Analytical phase

The analytical phase comprises of sample processing and the release of results.

Post-analytical phase

The post-analytical phase consists of diagnosis and evaluation of the results. The most important points include:

  • Correct allocation of the result to the patient
  • Plausibility check
  • Conveyance of critical results
  • Finally, medical interpretation using a combination of all results

Results from laboratory testing are not to be considered individually, but always in association with medical history and clinical examination!

Overview of important laboratory values

Current laboratory reference values can be found under “Tips and links” below!

Topics covered in this learning card

Laboratory values covered in other learning cards

Liver function tests

Parameters of hepatocellular damage

  • Relevance
    • Damage to hepatocytes results in the release of various enzymes that are detectable in blood and provide some indication of the severity of hepatic cell damage.
Laboratory parameter Physiological function Characteristics Frequent causes of elevation

Transaminases

Alanine aminotransferase (ALT)

Aspartate aminotransferase (AST)
Glutamate dehydrogenase (GLDH)
AST/ALT ratio < 0.7 (up to 1) = "inflammatory type" ≥ 1 = "necrotic type"
Possible causes
  • Uncomplicated viral hepatitis
  • Minor fatty liver disease
  • Extrahepatic cholestasis

Parameters of cholestasis

Laboratory parameter Physiological function Characteristics

Frequent causes of elevation

γ-glutamyl transpeptidase (γ-GT or GGT)
  • Membrane-bound enzyme of glutathione metabolism and amino acid transport
  • The most sensitive parameter for diseases of the liver and/or biliary tract!
  • Found in many tissues
Alkaline phosphatase (ALP)
  • Enzyme responsible for cleaving phosphate groups off various substances under alkaline conditions
  • Found in various isoenzymes in numerous tissues
Bilirubin Direct bilirubin
  • Bilirubin, which is conjugated by the liver, is water-soluble and is excreted by the gallbladder.
Indirect bilirubin

Parameters of hepatic synthesis

  • Relevance
    • The liver is one of the most important organs of protein synthesis.
    • The synthesis capacity of the liver can be reduced by the destruction of hepatocytes; (e.g., liver cirrhosis) or a deficiency of basic nutrients (e.g., in malnutrition).
    • In addition to the parameters listed in the table below, which provide an interpretation of the synthetic function of the liver, coagulation factors can also be used.
    • Vitamin K-dependent coagulation factors are particularly relevant, as their function is also indirectly reflected in the prothrombin time (PT, INR).
Laboratory parameter Physiological function Characteristics

Frequent causes of elevation

Frequent causes of decreased values
Albumin
  • Albumin makes up the majority of total plasma proteins, amounting to 60%. It has two main functions:
    • Maintenance of colloid osmotic pressure
    • Transport protein for degradation products and enzymes (such as indirect bilirubin)
Cholinesterase (CHE)

Kidney function tests

Parameters of renal function

  • Relevance
    • The kidney values listed here include substances that allow for an indirect interpretation of the glomerular filtration rate (GFR).
    • These parameters are usually determined in blood.
Laboratory parameter Physiological function Characteristics

Frequent causes of elevation

Creatinine

  • Creatinine is a constantly accumulating by-product of creatine catabolism with an unknown biological function.
  • The creatinine value is susceptible to interference.
Cystatin C
  • Cystatin C inhibits cysteine proteinases and is formed in the majority of nucleated cells.
  • Low susceptibility to interference

Other common renal substances

  • Relevance: Urea and uric acid are largely dependent on renal excretion; ; however, they provide an inaccurate assessment of renal function.

Laboratory parameter

Physiological function Characteristics

Frequent causes of elevation

Frequent causes of decreased values
Urea
  • Assessment of the metabolic state is possible.
  • Malnutrition
Uric acid
  • By-product of purine bases (component of DNA)

Pancreatic parameters

Laboratory parameter Physiological function Characteristics

Frequent causes of elevation

Frequent causes of decreased values
Pancreatic lipase
  • Enzyme involved in the digestion of fat in the small intestine.
Amylase
  • Enzyme involved in the digestion of carbohydrates in the small intestine.
Elastase In serum
  • Pancreas-specific enzyme
  • Most commonly used parameter for assessing exocrine pancreatic function
  • Reduced levels of elastase in serum are not diagnostically relevant.
In stool
  • Elevated levels of elastase in stool samples are not of diagnostic relevance.

Electrolytes

  • Relevance
    • Electrolytes are chemical compounds that disintegrate into ions in aqueous solutions.
    • May be differentiated into cations (positively charged) and anions (negatively charged).
    • The most important electrolytes are sodium, potassium, chloride, magnesium, phosphate, and hydrogen carbonate ions.
    • The electrolytes that most commonly cause pathological changes; (and are therefore standard in analysis) are sodium, potassium, and calcium; , which are discussed in detail in other learning cards (see disorders of sodium balance, disorders of potassium balance, disorders of calcium balance).

Chloride, magnesium, phosphate

Laboratory parameter Physiological function

Frequent causes of elevation

Frequent causes of decreased values Consequences of a shift
Chloride
  • Chloride is an anion and is the main counterpart of the positively charged sodium.
Magnesium
Phosphate

Iron metabolism

  • Relevance
    • Iron metabolism is closely associated with the formation of blood and the disease state of anemia.
    • In addition to the parameters listed here, hematological parameters such as hemoglobin, MCV, or MCH are important in assessing iron metabolism and identifying causes of anemia.
    • Iron deficiency is discussed in detail in a separate learning card (see iron deficiency).
Iron studies
Laboratory parameter Physiological function Characteristics

Frequent causes of elevation

Frequent causes of decreased values
Serum iron
  • Iron levels are subject to significant daily fluctuation.
Ferritin
  • Protein complex that is responsible for iron storage.
Transferrin*
  • Transport protein for iron
  • Transferrin saturation (TfS) can be calculated from both transferrin and iron values.
  • Anti-acute phase protein
*There are two additional forms of transferrin that may be used for diagnosis:

A decrease in ferritin with a reduced hemoglobin concentration is essentially evidence of iron deficiency anemia!

An elevated ferritin level does not exclude iron deficiency anemia. Concurrent chronic inflammation can increase ferritin levels!

  • Herold G. Internal Medicine. Cologne, Germany: Herold G; 2014.
last updated 11/22/2018
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