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Monday, August 11, 2025

In vivo diagnostic biologicals


1. Definition

  • Biological products administered directly into the body (in vivo) to help diagnose diseases or assess physiological functions

  • Work by producing a measurable or visible reaction in the patient’s body

  • Differ from in vitro diagnostics, which test samples outside the body (e.g., in laboratory assays)

2. General Characteristics

  • Derived from biological sources such as proteins, antibodies, antigens, enzymes, hormones, or microorganisms

  • Usually administered by injection, inhalation, or topical application depending on the diagnostic purpose

  • Used to detect immune responses, infections, allergic sensitivities, or metabolic activity

3. Mechanism of Action

  • Immune reaction-based: The product interacts with the immune system, producing a detectable local or systemic response (e.g., swelling, redness, antibody binding)

  • Physiological function testing: Mimics or stimulates natural processes, allowing clinicians to measure functional capacity (e.g., hormone stimulation tests)

  • Pathogen detection: Exposes the body to harmless antigenic components of a suspected pathogen and observes response

4. Types of In Vivo Diagnostic Biologicals

A. Allergy Diagnostics

  • Allergen extracts – proteins from pollens, dust mites, animal dander, insect venom, foods

  • Used in skin prick tests or intradermal tests to detect allergic hypersensitivity

B. Infectious Disease Diagnostics

  • Tuberculin purified protein derivative (PPD) – detects latent Mycobacterium tuberculosis infection (Mantoux test)

  • Coccidioidin & histoplasmin skin tests – identify exposure to fungal pathogens (limited use)

  • Tularin – diagnostic for tularemia in specific research settings

C. Immune Function Assessment

  • Candida, mumps, or trichophyton antigens – used in delayed-type hypersensitivity testing to assess T-cell mediated immunity

D. Hormonal & Endocrine Function Tests

  • ACTH (cosyntropin) stimulation test – assesses adrenal cortex function in suspected adrenal insufficiency

  • TRH stimulation test – evaluates pituitary TSH secretion

  • GnRH stimulation test – used in reproductive hormone evaluation

E. Other Specialized Tests

  • Bacillus Calmette-Guérin (BCG) as a diagnostic tool – occasionally used to evaluate bladder immune response in oncology trials

  • Schilling test using radiolabeled vitamin B12 and intrinsic factor (historical; now largely replaced by other diagnostics)

5. Administration Routes

  • Intradermal – most common for immune-based skin testing (e.g., PPD, allergy tests)

  • Subcutaneous or intramuscular – some hormonal function tests

  • Intravenous – specialized endocrine stimulation tests

  • Topical or conjunctival – limited ophthalmic allergy testing

6. Interpretation of Results

  • Qualitative – presence or absence of reaction (positive/negative)

  • Quantitative – size of induration, hormone level changes, or other measurable biological markers

  • Requires standardization in measurement to avoid false results

7. Advantages

  • Directly measures body’s physiological or immune response

  • Can detect conditions not easily identified by laboratory testing alone

  • Often low-cost and simple to administer in clinical settings

8. Limitations

  • Risk of false positives due to cross-reactivity with other antigens

  • False negatives in immunocompromised patients due to lack of immune response

  • Requires trained personnel for administration and interpretation

  • Some tests are time-sensitive for accurate reading (e.g., PPD within 48–72 hours)

9. Adverse Reactions

  • Local reactions: redness, swelling, pruritus, tenderness at injection site

  • Systemic reactions: rare, may include anaphylaxis (especially with allergen extracts)

  • Delayed hypersensitivity reactions in immune-based tests

  • Pain or discomfort at administration site

10. Contraindications

  • Severe allergy to the test material

  • Severe dermatological conditions interfering with skin test reading

  • Acute illness or fever in some immune function tests

  • Pregnancy: some hormonal challenge tests may be contraindicated

11. Precautions

  • Emergency resuscitation equipment should be available for tests with allergenic potential

  • Avoid in patients on systemic corticosteroids or immunosuppressive therapy unless medically justified

  • Document patient’s baseline medical history to reduce interpretation errors

12. Drug Interactions

  • Immunosuppressants (e.g., corticosteroids, chemotherapy) – may blunt immune test responses

  • Antihistamines – can reduce skin reactivity in allergy testing

  • Recent live vaccines – may interfere with delayed-type hypersensitivity skin tests

13. Examples of In Vivo Diagnostic Biologicals (Selected)

  • Tuberculin PPD

  • Candida albicans skin test antigen

  • Mumps skin test antigen

  • Allergen extracts (dust mite, cat dander, ragweed)

  • Cosyntropin (synthetic ACTH)

  • Gonadorelin (GnRH) for pituitary testing

  • Thyrotropin-releasing hormone (TRH)




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