• Clinical science

Laboratory medicine

Summary

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 NBME laboratory reference values can be found under “Tips & Links” below.

Overview of important laboratory values

Topics covered in this learning card

Laboratory values covered in other learning cards

Inflammatory markers

Inflammatory markers are of great diagnostic value because their plasma concentrations change in different ways depending on the underlying cause.

Erythrocyte sedimentation rate (ESR)

Acute phase reaction

Important acute phase reactants

To memorize the positive acute phase reactants, think More FFiSH in the C (sea): More = upregulation, F = Ferritin, Fi = Fibrinogen, S = Serum amyloid A, H = Haptoglobin, C = C-reactive protein.

References:[1][2][3][4][5][6][7][8]

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

Parameters of cholestasis

Laboratory parameter Physiological function Characteristics

Frequent causes of elevation

γ-glutamyl transpeptidase (γ-GT or GGT)
  • The most sensitive parameter for diseases of the liver and/or biliary tract!
  • Found in many tissues
  • Cholestasis (obstructive or nonobstructive)
  • Alcohol use
Alkaline phosphatase (ALP)
  • Enzyme responsible for cleaving phosphate groups off various substances under alkaline conditions
  • Found in various isoenzymes in numerous tissues
Bilirubin Direct (conjugated) bilirubin
  • Bilirubin, which has been conjugated with glucuronic acid in the liver, is water-soluble and is excreted by the gallbladder.
Indirect (unconjugated) 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 elevated values

Frequent causes of decreased values
Albumin
Cholinesterase (CHE)

Kidney function tests

Parameters of renal function

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

Frequent causes of elevation

Creatinine

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

Other common renal substances

Laboratory parameter

Physiological function Characteristics

Frequent causes of elevation

Frequent causes of decreased values
Urea
  • Assessment of the metabolic state is possible.
Uric acid
  • Triggers gout
  • Hyperuricosuria can lead to uric acid nephropathy.

Pancreatic parameters

  • Relevance
Laboratory parameter Physiological function Characteristics

Frequent causes of elevation

Frequent causes of decreased values
Pancreatic lipase
Amylase
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

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

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
Transferrin*
  • Transport protein for iron
  • Transferrin saturation (TfS) can be calculated from both transferrin and iron values.
    • A high TfS value is an indication of iron overload
    • A low TfS value indicates iron deficiency.
  • 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!