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Laboratory medicine

Last updated: April 8, 2021

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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 article covers important laboratory parameters such as liver function tests and iron metabolism. Further parameters of clinical relevance may be found in other articles and are listed in the section “Overview of important laboratory values”. Current NBME laboratory reference values can be found under “Tips & Links” below.

Covered in this article

Covered in other articles

Complete blood count (CBC)

RBC parameters

Overview of RBC parameters
Parameter Reference range Description Common causes of elevation Common causes of reduction
RBC count
  • : 4.3–5.9 million/mm3 (4.3–5.9 x 1012/L)
  • : 3.5–5.5 million/mm3 (3.5–5.5 x 1012/L)
  • Absolute number of RBCs contained in a certain volume of blood
Hemoglobin (Hb)
  • : 13.5–17.5 g/dL (2.09–2.71 mmol/L)
  • : 12–16 g/dL (1.86–2.48 mmol/L)
Hematocrit (Hct)
  • : 41%–53% (0.41–0.53)
  • : 36%–46% (0.36–0.46)
  • Ratio of RBC volume to total blood volume
Mean corpuscular volume (MCV)
  • 80–100 μm3 (80–100 fL)
  • Average volume of an RBC
Mean corpuscular hemoglobin (MCH)
  • 25.4–34.6 pg/cell (0.39–0.54 fmol/cell)
Mean corpuscular hemoglobin concentration (MCHC)
  • 31–36% Hb/cell (4.81-5.58 mmol Hb/L)

Reticulocyte count

  • 0.5–1.5% (0.005–0.015)

Absolute reticulocyte count

  • 50–100 x 109/L [1]

Corrected reticulocyte count

  • No clinical relevance in healthy individuals

Reticulocyte production index (RPI)

  • Individuals without anemia: 1
  • Individuals with anemia [2]
    • ≥ 2 is a normal value.
    • < 2 is an abnormal value.
Red blood cell distribution width (RDW)
  • 11–15% [3]
  • Measures the variation in RBC volumes

WBC parameters

WBC count and differential [4]

Overview of WBC parameters

Parameter

Reference range

Common cause of elevation

Common cause of reduction
WBC count
  • 4,500–11,000/mm3 (4.5–11.0 x 109/L)
Segmented neutrophil count
  • 54–62% (0.54–0.62)
Band neutrophil count
  • 3–5% (0.03–0.05)
Eosinophil count
  • 1–3% (0.01–0.03)
Basophil count
  • 0–0.75% (0–0.0075)
  • Basophilia: > 0.75%
  • Causes
    • Often occurs together with eosinophilia (e.g., in allergic responses)
    • CML
  • Basopenia: difficult to assess because the normal basophil count is already very low
Lymphocyte count
  • 25–33% (0.25–0.33)
Monocyte count
  • 3–7% (0.03–0.07)

For causes of eosinophilia, think CHINAA: Collagen vascular disease (e.g., eosinophilic granulomatosis), Helminths, Hyper-IgE syndrome, Neoplasms, Allergies, Addison disease.

Neutrophil left shift

Leukemoid reaction [13]

Leukoerythroblastic reaction

Platelet count

Pancytopenia

Peripheral blood smear

Overview of iron studies
Laboratory parameter Reference range Description

Common causes of elevation

Common causes of reduction
Serum iron
  • Male: 65–175 μg/dL
  • Female: 50–170 μg/dL
  • Measurement of free, circulating iron
  • Serum iron levels are subject to physiological daily fluctuation and can be influenced by dietary intake.
Ferritin
  • Male: 20–250 ng/mL
  • Female: 10–120 ng/mL
  • Main storage protein for iron
Transferrin*
  • 200–360 mg/dL

Total iron binding capacity (TIBC)

  • 250–400 μg/dL
  • Indirect measure of transferrin (same diagnostic value)
Transferrin saturation (TfS)
  • 20–50%
Soluble transferrin receptor (sTfR)
  • Male: 2–5 mg/L
  • Female: 1.9–4.4 mg/L
  • Part of the transferring receptor of erythroid precursor cells of the bone marrow
  • Serum levels reflect the erythropoiesis rate.
* There are two additional forms of transferrin that may be used for diagnosis: beta-2 transferrin (used for the detection of a CSF leak in skull fractures) and carbohydrate-deficient transferrin (used to detect heavy alcohol consumption).

In combination with an elevated TIBC, low hemoglobin, ferritin, and iron levels are diagnostic of iron deficiency anemia.

Elevated ferritin levels do not rule out iron deficiency anemia. Ferritin can be elevated in response to simultaneous chronic inflammation.

Most common tests

Overview of coagulation parameters [14]

Parameter

Reference range

Description Pathways tested Clinical relevance
Common Intrinsic Extrinsic
Prothrombin time (PT)
  • 11–15 seconds
International normalized ratio (INR)
  • 0.9–1.1 [15]
  • Derived from PT, but calculated by comparing the laboratory-specific PT to a standardized PT
(Activated) partial thromboplastin time (aPTT, PTT)
  • 25–40 seconds
  • Time it takes to produce fibrin polymers after adding phospholipids
(Plasma) thrombin time (TT)
  • < 2 seconds deviation from control
Reptilase time
  • 14–24 seconds [16]
  • Time it takes to produce fibrin polymers after adding reptilase

Additional tests

Parameters of hepatocellular damage

  • Damage to hepatocytes results in the release of various enzymes that are then detectable in the blood.
  • These parameters may help to evaluate the cause and severity of hepatic cell damage.
Laboratory parameters of hepatocellular damage
Laboratory parameter Physiological function Characteristics Common causes of elevation

Transaminases (aminotransferases)

Alanine aminotransferase (ALT)

Aspartate aminotransferase (AST)

Glutamate dehydrogenase (GLDH)

AST/ALT ratio

Parameters of cholestasis

Laboratory parameters of cholestasis
Laboratory parameter Physiological function Characteristics

Common causes of elevation

Alkaline phosphatase (ALP)
  • Enzyme that cleaves phosphate groups under alkaline conditions
γ-Glutamyl transpeptidase (γ-GT, GGT) [18]
  • The most sensitive parameter for diseases of the liver and/or biliary tract
  • Usually the first liver enzyme to rise after bile duct obstruction
  • Used to confirm hepatic origin of elevated ALP levels
  • Cholestasis (obstructive or nonobstructive)
  • Alcohol use
  • Not elevated in bone disease (in contrast to ALP)
Bilirubin Indirect (unconjugated) bilirubin
  • Water-insoluble
Direct (conjugated) bilirubin
  • Water-soluble

5'-nucleotidase (5'-NT) [20]

  • Used to confirm hepatobiliary origin of elevated ALP levels


Indirect bilirubin is water-insoluble.

Parameters of hepatic synthesis

  • Liver dysfunction can reduce the hepatic production of various substances, which is reflected in their decreased serum levels.
  • The initial evaluation of liver synthesis capacity typically consists of determining serum albumin, PT/INR, and platelet count.
  • For a more exhaustive list of substances produced by the liver, see the article on the “Liver.”
Laboratory parameters of hepatic synthesis
Laboratory parameter Physiological function Characteristics

Common causes of elevation

Common causes of reduction
Albumin

PT and INR

  • Tests that evaluate the common and extrinsic pathways of coagulation
  • Usually no clinical relevance

Platelet count

  • This table lists parameters that are commonly tested to evaluate pancreas function and disease. For a more exhaustive list of substances produced by the pancreas, see the article “Pancreas.”
Overview of pancreatic parameters
Laboratory parameter Physiological function Characteristics

Common causes of elevation

Common causes of reduction
Pancreatic lipase
Amylase
  • Several isoenzymes are found in many different tissues (not specific to pancreas).
  • Mainly produced by the pancreas and salivary glands
Elastase In serum
  • Usually no clinical relevance
In stool
  • Usually no clinical relevance

Chloride (Cl-)

  • Reference range: 95–105 mg/dL (95–105 mmol/L)
  • Physiology
    • Cl- is the main anion in the extracellular space and is the counterpart to the main extracellular cation Na+.
      • Changes in serum chloride levels typically reflect changes in serum sodium levels.
      • An exception to this is in acid-base disorders, which can be caused by or lead to changes in chloride levels independent of sodium.
    • Cl- is involved in water balance, maintaining osmotic pressure, and acid-base balance.

Disorders of chloride balance

Common causes

Clinical features

Hyperchloremia

(Cl- > 105 mg/dL)

  • Clinical features of the underlying cause
  • Potentially, symptoms of accompanying electrolyte imbalances (e.g., hypernatremia or hyponatremia)

Hypochloremia

(Cl- < 95 mg/dL)

Magnesium (Mg2+)

For more details regarding hypomagnesemia, see the article “Hypomagnesemia.”

Hypermagnesemia

Phosphate (H2PO4-, HPO42-)

For more on the effects and treatment of low phosphate levels, see “Hypophosphatemia.”

Hyperphosphatemia

General

Hyperlactatemia

Inflammatory markers are used in the diagnosis and monitoring of a large spectrum of conditions, including infection, autoimmune diseases, and malignancies. Most inflammatory processes lead to elevated CRP, elevated ESR, and leukocytosis.

Acute phase reaction [24][25]

Positive acute phase reactants

Hepatic production and serum levels of positive acute phase reactants increase in response to inflammatory processes.

Important positive acute phase reactants: “Upstream, Fred Hopes He Catches Some Perfect Fish.” (Upregulation, Ferritin, Haptoglobin, Hepcidin, C-reactive protein, Serum amyloid A, Procalcitonin, Fibrinogen)

Negative acute phase reactants

Serum levels of negative acute phase reactants decrease in response to inflammatory processes.

Erythrocyte sedimentation rate (ESR) [30]

White blood cell count

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  36. LABORATORY VALUES.