Laboratory medicine

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

• Hematological parameters
• Iron studies
• Coagulation studies
• Liver function tests
• Pancreatic parameters
• Other metabolic parameters
• Inflammatory markers

Covered in other articles

• Cardiac parameters
  • Cardiac biomarkers
  • BNP and NT-proBNP
• Thyroid parameters: See “Thyroid function tests.”
• Kidney parameters
  • Renal function tests
  • Urinalysis
• Carbohydrate metabolism parameters: fasting plasma glucose, HbA1c, OGTT, and C-peptide (see “Diagnostics” in “Diabetes mellitus”)
• Lipid parameters
  • Parameters of fat metabolism
  • Changes in lipid parameters
• Electrolytes
  • Sodium: See “Hypernatremia” and “Hyponatremia.”
  • Potassium: See “Hyperkalemia” and “Hypokalemia.”
  • Calcium: See “Hypocalcemia” and “Hypercalcemia.”
• Arterial blood gas analysis
• Bicarbonate and pH: See “Acid-base disorders.”
• Cerebrospinal fluid analysis
• Tumor markers

Complete blood count (CBC)

• A laboratory test that measures:
  • Red blood cell (RBC) count
  • Hemoglobin (Hb)
  • Hematocrit (Hct)
  • Mean corpuscular volume (MCV)
  • Mean corpuscular hemoglobin (MCH)
  • Mean corpuscular hemoglobin concentration (MCHC)
  • White blood cell (WBC) count
  • Platelet count
• In a CBC with differential, the percentage of WBC subtypes (e.g., neutrophils, lymphocytes, etc.) is also included.

RBC parameters

• RBC indices
  • A set of calculated RBC parameters that comprises MCV, MCH, MCHC, and RBC distribution width
  • Used to determine the cause of anemia

WBC parameters

WBC count and differential ^[4]

• Granulocytes make up the largest portion of WBCs: 1,500–8,500/μL. ^[5]
• An increase (granulocytosis) or decrease (granulocytopenia) in granulocytes (especially neutrophils) usually contributes the most to changes in the total WBC count.
• For more information about the structure and functions of particular types of WBCs, see the section “White cell line - leukocytes” in “Basics of hematology.”

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

Neutrophil left shift

• Definition: increase in immature leukocytes (e.g., band cells, metamyelocytes) in the peripheral blood
• Causes
  • Infections (especially bacterial infections)
  • Acute inflammation
  • Severe anemia
  • Bone marrow infiltration
  • Necrosis

Leukemoid reaction ^[13]

• Definition
  • Reactive leukocytosis: cell hyperplasia with proliferation of myeloid or lymphoid elements
  • Characterized by an increased leukocyte alkaline phosphatase (LAP) score
• Causes
  • Infections (predominantly bacterial, e.g., pertussis)
  • Severe purulent conditions (e.g., perforated appendicitis)
  • Drugs (e.g., steroids)
  • Associated with certain solid tumors (e.g., lung cancer, renal cancer)
  • Mimics hematopoietic malignancy

Leukoerythroblastic reaction

• Definition: The presence of immature and/or abnormal cells : (e.g.,; dacrocytes, nucleated red blood cells; , myelocytes, metamyelocytes, and myeloblasts, giant platelets) in the peripheral blood.
• Causes: Bone marrow infiltration or fibrosis (e.g., myelofibrosis, bone marrow metastasis)

Platelet count

• Reference range: 150,000–400,000/mm³ (150–400 x 10⁹/L)
• Thrombocytosis
  • Definition: absolute platelet count of > 400,000/mm³
  • Causes
    • Reactive thrombocytosis : secondary to certain conditions, e.g.:
      • Malignancy (e.g., CML)
      • Splenectomy
      • Chronic inflammation
        • Autoimmune diseases: e.g., rheumatoid arthritis, celiac disease, connective tissue disorders
        • Chronic infections: e.g., tuberculosis, syphilis
      • Anemia: hemolytic anemia, iron deficiency
      • Increased cellular turnover following acute blood loss
      • Pregnancy
      • Following cytostatic therapy
    • Splenectomy
    • Essential thrombocythemia
• Thrombocytopenia
  • Definition: absolute platelet count of < 150,000/mm³
  • Causes: See “Etiology” in “Thrombocytopenia.”

Pancytopenia

• Definition: a decrease in the number of cells of all cell lines (i.e., RBCs, WBCs, and platelets) in the peripheral blood
• Causes
  • Aplastic anemia
  • Fanconi anemia
  • Multiple myelomas
  • Myelodysplastic syndrome
  • Acute and chronic leukemia
  • Chemotherapy
  • Autoimmune disease (e.g., SLE)
  • Infections (e.g., CMV, EBV)

Peripheral blood smear

• Manual examination of a peripheral blood sample under a microscope
• May reveal pathognomonic RBC morphologies, which can be used to identify certain types of anemia that automated RBC indices cannot
• Examples of findings
  • Abnormal cell shapes
    • E.g., schistocytes, spherocytes, sickle cells, macrocytes
    • See also “Anemia” and “Erythrocyte morphology.”
  • Inclusion bodies (e.g., Howell-Jolly bodies, basophilic stippling, Heinz bodies)
  • Detection of pathogens (e.g., Plasmodium spp.)

• Iron is a trace element that is essential for hematopoiesis.
  • Iron is required for the synthesis of heme and, subsequently, of hemoglobin.
  • See “Iron” in “Trace elements.”
• Iron studies are particularly important in the differential diagnosis of anemia.
  • See also “Hematological parameters” above.
  • See the articles “Anemia” and “Iron deficiency anemia.”

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

Additional tests

• Bleeding time: amount of time it takes for a bleeding to stop after a small skin puncture
  • Prolonged in disorders of primary hemostasis (thrombocytopenia and platelet dysfunction)
  • Not routinely performed ^[17]
• Anti-factor Xa assay: used to monitor certain anticoagulant therapies
  • Low molecular weight heparin (LMWH) or fondaparinux therapy in patients with impaired renal function
  • Unfractionated heparin (UFH) therapy if aPTT is inconclusive (e.g., due to coagulation factor deficiency or heparin antibodies)
• D-dimer: fibrin degradation product that correlates with activity of coagulation and fibrinolysis
  • High sensitivity: Increased serum D-dimer levels occur in deep vein thrombosis (DVT), pulmonary embolism (PE), and disseminated intravascular coagulation.
  • Low specificity: Elevation can also occur due to other conditions, including malignancies, infection, pregnancy, renal insufficiency, or surgical procedures.
• Ristocetin cofactor assay: measures the ability of von Willebrand factor (vWF) to agglutinate platelets
  • Ristocetin: antibiotic that activates vWF, which then binds to glycoprotein Ib on platelets
  • Interpretation: Failure of platelet aggregation or a ristocetin cofactor level < 30 IU/dL occurs in von Willebrand disease and Bernard-Soulier syndrome.
• Coagulation factor assays: e.g., reduced factor VIII activity in hemophilia A
• Clot observation test: simple method of point-of-care testing to assess coagulation, especially if hyperfibrinolysis is suspected
  • Procedure: A small amount of blood is collected in a test tube and examined macroscopically after a few minutes.
  • Interpretation
    • Stable thrombus formation : normal coagulation
    • No thrombi formation: severely impaired coagulation (e.g., in DIC)
    • Formation of an unstable thrombus that dissolves quickly: hyperfibrinolysis
• Thromboelastography: full blood assay that assesses the formation, stability, and dissolution of a thrombus alongside PT and aPTT
• Rumpel-Leede test: detects primary hemostasis disorders and increased dermal capillary fragility
  • Procedure: A tourniquet is applied to the upper arm and the cuff is inflated to a pressure that is midway between the patient's systolic and diastolic blood pressure. After 5 minutes, the tourniquet is removed and the cubital pit and forearm are examined for petechiae.
  • Interpretation
    • Test is positive if ≥ 10 petechiae appear per square inch.
    • Positive test results occur in:
      • Dengue hemorrhagic fever
      • Disorders affecting primary hemostasis (e.g., thrombocytopenia)
      • Increased capillary fragility (e.g., prolonged steroid use, scurvy)
      • Vascular hemorrhagic diathesis (Osler-Weber-Rendu disease, Henoch-Schonlein purpura)
      • Scarlet fever

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.

AST/ALT ratio

• The ratio of AST serum levels to ALT serum levels is used to determine the etiology of hepatocellular injury.
• AST/ALT < 1 (AST < ALT)
  • Uncomplicated viral hepatitis
  • Minor fatty liver disease
  • Extrahepatic cholestasis
• AST/ALT ≥ 1 (AST > ALT)
  • Alcoholic hepatitis
    • Typically AST/ALT > 2
    • AST usually does not exceed 500 U/L in alcoholic hepatitis.
  • Fulminant, necrotic hepatitis
  • (Decompensated) cirrhosis: The AST/ALT ratio can increase as fibrosis advances.
  • Hepatocellular carcinoma, liver metastases
  • Muscle damage
  • Myocardial infarction

Parameters of cholestasis

• Cholestasis typically leads to an increase in direct (conjugated) bilirubin and induces the production of ALP and γ-GT.
• Fractionating bilirubin (i.e., determining total bilirubin and direct bilirubin) can help to differentiate between cholestasis and other causes of hyperbilirubinemia.
• In cholestasis, the retention of bile salts can additionally lead to hepatocyte damage, resulting in the release of various enzymes (see “Parameters of hepatocellular damage”).
• For more information, see “Jaundice and cholestasis.”

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.”

• 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.”

• Basic metabolic panel (BMP) measures the serum concentrations of:
  • Electrolytes (typically Na⁺, K⁺, Cl^-)
  • Bicarbonate
  • Blood urea nitrogen
  • Creatinine
  • Glucose
• Comprehensive metabolic panel (CMP) measures all parameters included in the BMP plus:
  • Calcium
  • Total serum protein and albumin
  • ALT, AST, AP, and bilirubin
• Clinical use: Metabolic parameters are used to diagnose and monitor a wide variety of conditions. They are particularly useful in the diagnosis of:
  • Acid-base disorders
  • Kidney failure
  • Hypertension
  • Congestive heart failure
  • Diabetes mellitus

• Electrolytes develop when a salt dissolves in a solution, causing it to separate into cations (positively charged) and anions (negatively charged).
• Normal body function requires the appropriate concentrations of the following electrolytes: sodium (Na⁺), potassium (K⁺), chloride (Cl^-), bicarbonate (HCO₃^-), calcium (Ca²⁺), magnesium (Mg²⁺), and phosphate (H₂PO₄^-, HPO₄^2-).
• More detailed information about electrolyte imbalances can be found in dedicated articles.
  • “Hypernatremia” and “Hyponatremia”
  • “Hyperkalemia” and “Hypokalemia”
  • “Hypercalcemia” and “Hypocalcemia”
  • “Hypomagnesemia”
  • “Hypophosphatemia”
  • “Electrolyte repletion”
  • “Acid-base disorders”

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.

Magnesium (Mg²⁺)

• Reference range: 1.5–2.0 mg/dL (0.75–1.0 mmol/L)
• Physiology
  • Part of bone matrix
  • Mg²⁺ is one of the main intracellular cations.
  • Magnesium balance is closely interlinked with calcium and potassium homeostasis.
    • An imbalance of one of these electrolytes may be a result of or cause an imbalance in another.
    • Magnesium prevents the intracellular accumulation of calcium. A balance of intracellular Mg²⁺ and extracellular Ca²⁺ is required to maintain normal neuromuscular activity.
    • Magnesium repletion is required for effective treatment of hypocalcemia or hypokalemia.
  • Mg²⁺ is an important cofactor for a large number of enzymes and transporters.
  • Regulation of magnesium homeostasis
    • Reabsorption rates in the kidneys respond to magnesium serum levels.
    • Influenced by a variety of hormones (e.g., PTH, insulin, glucagon, PTH, calcitonin, ADH, corticosteroids)

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

Hypermagnesemia

• Common causes
  • Renal failure
  • Increased tissue breakdown
    • Rhabdomyolysis
    • Trauma (e.g., burns, surgery)
  • Increased magnesium intake
    • Magnesium therapy (e.g., eclampsia, premature labor)
    • Overdose of drugs containing magnesium (e.g., laxatives, magnesium hydroxide)
    • Dietary oversupplementation
  • Adrenal insufficiency (e.g., Addison disease)
  • Severe hypovolemia
  • Hypothyroidism
• Clinical features
  • Mild hypermagnesemia (4–6 mg/dL): often asymptomatic
    • Nausea
    • Lethargy
    • Reduced deep tendon reflexes
    • Blurry vision
  • Moderate hypermagnesemia (6–10 mg/dL)
    • Cardiovascular symptoms
      • Hypotension
      • Bradycardia
      • ↑ PR interval
      • ↑ QRS duration
      • ↑ QT interval
    • Somnolence
    • Blurry vision
    • Hypocalcemia
  • Severe hypermagnesemia (> 10 mg/dL)
    • Muscle paralysis (flaccid quadriplegia)
    • Cardiac arrest
    • Respiratory failure
• Treatment
  • Discontinue magnesium-containing medication
  • IV isotonic fluids (e.g., normal saline)
  • Loop diuretics (e.g., furosemide)
  • Calcium gluconate
  • Consider dialysis (especially in severe cases of hypermagnesemia in patients with renal failure).
  • Treat the underlying cause.

Phosphate (H₂PO₄^-, HPO₄^2-)

• Normal range: 3.0–4.5 mg/dL (1.0–1.5 mmol/L)
• Physiology
  • Approx. 85% of the body's phosphate is found in the bone matrix. ^[22]
  • Outside of bone, phosphate is mainly found in the intracellular space (esp. in soft tissue cells).
  • Component of many important molecules, including creatine phosphate, membrane phospholipids, DNA, ATP/ADP, 2,3-DPG, and NADP
  • Phosphate homeostasis
    • Vitamin D stimulates intestinal absorption and release of phosphate from bones.
    • PTH stimulates renal phosphate excretion by inhibiting its reabsorption in the kidneys.

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

Hyperphosphatemia

• Common causes
  • Renal failure
  • Hypoparathyroidism
  • Pseudohypoparathyroidism
  • Vitamin D intoxication
  • Increased tissue breakdown (e.g., tumor lysis syndrome, rhabdomyolysis, crush injury)
  • Increased phosphate intake (e.g., phosphate-containing enemas)
  • Bisphosphonates (have also been shown to cause hypophosphatemia) ^[23]
• Clinical features
  • Often asymptomatic
  • High PO₄^3- levels cause the formation of an insoluble compound with calcium, which can lead to:
    • Hypocalcemia
    • Nephrolithiasis
    • Calcifications in the skin
• Treatment
  • Treat the underlying cause.
  • Discontinue phosphate intake (dietary or medication).
  • Give phosphate binders (e.g., aluminium hydroxide, calcium carbonate).
  • Consider dialysis (especially in severe cases of hyperphosphatemia in patients with renal failure).

General

• Reference range: < 2.0 mmol/L
• Physiology
  • Lactic acid is the end product of anaerobic glycolysis, which is accomplished in the Cori cycle.
  • The majority of lactic acid is produced by muscle cells and RBCs, especially in hypoxic states.

Hyperlactatemia

• Common causes
  • Tissue hypoperfusion (hypoxic states)
    • Heart failure
    • Sepsis and other infections
    • Shock
    • Infarction
    • Lung disease (e.g., pulmonary embolism)
  • Dehydration
  • Severe anemia and pancytopenia
  • Poisoning
    • CO (carbon monoxide)
    • Ethanol
    • Methanol
    • Ethylene glycol
  • Liver disease
  • Drugs (e.g., metformin or isoniazid)
  • Thiamine deficiency
  • Kidney disease (especially in individuals with diabetes)
  • Strenuous exercise
• Clinical features: Symptoms are largely caused by lactic acidosis and compensatory mechanisms. They can include:
  • Tachypnea
  • Tachycardia
  • Nausea, vomiting
  • Sweating
  • Confusion

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]

• Definition: systemic response to systemic and/or local disturbances (e.g., acute or chronic inflammation, infection, surgery, trauma, malignancy)
  • Part of the humoral immune response of the innate immune system
  • Consists of:
    • Fever
    • Leukocytosis
    • Increased synthesis of > 30 acute phase reactants
      • Produced by the liver
      • Synthesis is induced mainly by IL-6.
• Diagnostic use
  • Serum levels of acute phase reactants are used to detect and monitor inflammatory processes.
  • An increase of acute phase reactants is reflected in an increase in alpha-1 and alpha-2 zones in serum protein electrophoresis.

Positive acute phase reactants

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

• C-reactive protein (CRP)
  • Promotes the opsonization of pathogens, which leads to increased phagocytosis by macrophages
  • Activates the complement system
  • High sensitivity for detecting inflammation but not specific to any disease or organ
  • Increases 6–12 hours after the inflammatory process begins
  • Half-life is 24 hours.
• Procalcitonin
  • Sensitive parameter for monitoring the progression of bacterial infections, especially pneumonia and sepsis
  • Peptide precursor of calcitonin
• Ferritin ^[26][27]
  • Serum ferritin levels increase in infection to limit the amount of free iron available to pathogens, as well as in malignancy to limit the amount available to tumor cells.
  • In contrast, some organisms (e.g., Pseudomonas) cause serum ferritin levels to drop.
  • See “Iron studies.”
• Hepcidin
  • Reduces iron available to pathogens by:
    • Decreasing intestinal iron absorption (via ferroportin degradation)
    • Preventing the release of iron from macrophages
  • Can cause anemia of chronic disease
• Haptoglobin: binds free hemoglobin
  • Antimicrobial effects: In infection, haptoglobin is upregulated to make extracellular heme iron less available to pathogens.
  • Antioxidative effects
    • Free hemoglobin (e.g. in hemolysis) can cause oxidative damage.
    • Haptoglobin levels decrease in hemolysis.
• Serum amyloid A (SAA)
  • Recruits immune cells to inflammatory sites
  • Prolonged increased levels may cause amyloidosis.
• Fibrinogen
  • Coagulant that promotes wound healing and endothelial repair
  • Correlates with ESR
• Von Willebrand factor ^[28]
• α1-antitrypsin: protection from protease activity
• Interleukin-6 (IL-6)
• Ceruloplasmin: binds free iron and has antioxidative effects ^[29]
• Complement components

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.

• Albumin: Reduced production of albumin conserves amino acids that can then be used to produce positive acute phase reactants.
• Transferrin: Macrophages take up transferrin and use it to remove iron from circulation, making it unavailable for pathogens.
• Antithrombin
• Transthyretin

Erythrocyte sedimentation rate (ESR) ^[30]

• Description: distance that erythrocytes (RBCs) descend in a vertical tube of anticoagulated blood over one hour
• Normal ranges
  • ♀ 0–20 mm/h
  • ♂ 0–15 mm/h
• Common causes of increase
  • All conditions associated with ↑ fibrinogen (e.g., infection, inflammation, malignancies)
    • Normally, RBCs are separated from each other by their negatively charged surfaces.
    • Elevated fibrinogen levels lead to a decrease in the negative charges, causing RBCs to aggregate and sink faster in the test tube.
  • Inflammation
  • Infection
  • Malignancies (e.g., multiple myeloma, Waldenstrom macroglobulinemia, metastases)
  • Autoimmune diseases; (e.g., SLE, rheumatoid arthritis, giant cell arteritis, polymyalgia rheumatica, de Quervain thyroiditis)
  • Anemia
  • Macrocytosis
  • Renal disease (e.g., nephrotic syndrome, ESRD)
  • Pregnancy (leads to ↑ fibrinogen)
  • Old age
• Common causes of decrease
  • Conditions affecting RBCs
    • Polycythemia (the increased number of RBCs lowers the concentration of aggregation factors)
    • Sickle cell disease (irregular and smaller RBCs sink slower)
    • Spherocytosis
    • Microcytosis
  • Leukocytosis with very high WBC count (e.g., in chronic lymphocytic leukemia)
  • Congestive heart failure (CHF) ^[31]
  • Hypofibrinogenemia (e.g., in DIC)
  • Hypogammaglobulinemia

White blood cell count

• Inflammation typically causes leukocytosis (> 11,000/mm³).
• Which type of WBC is increased depends on the underlying cause.
• See “Leukocytosis” and “Leukemoid reaction” above.

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36. LABORATORY VALUES.