Evaluation of Drugs: Evaluation of drugs means the process of determining the identity, purity, quality, strength, safety and efficacy of a drug. In pharmacognosy, evaluation is especially important for crude drugs of natural origin because their quality can vary depending on the plant source, geographical area, collection time, storage conditions, and processing.
The main methods of drug evaluation are:
- Organoleptic evaluation
- Microscopic evaluation
- Qualitative microscopy
- Quantitative microscopy
- Physical evaluation
- Chemical evaluation
- Biological evaluation
Evaluation of Drugs
1. Organoleptic Evaluation
Organoleptic evaluation is the evaluation of a drug using the sense organs. It is also called sensory evaluation.
The drug is examined based on its:
- Colour
- Odour
- Taste
- Size
- Shape
- Surface
- Texture
- Appearance
- Fracture
This is one of the simplest and quickest methods of crude drug evaluation.
A. Colour
The colour of the drug is observed and compared with the standard drug.
Example:
- Clove – dark brown
- Senna leaves – greenish to yellowish-green
- Turmeric – yellow to orange-yellow
A change in colour may indicate poor storage, ageing, deterioration or adulteration.
B. Odour
The characteristic smell of a drug is determined.
Examples:
- Clove – characteristic aromatic odour
- Fennel – aromatic
- Asafoetida – strong characteristic odour
Absence or change in odour may indicate deterioration or adulteration.
C. Taste
Taste may be:
- Sweet
- Bitter
- Salty
- Sour
- Pungent
- Astringent
Examples:
- Nux vomica – intensely bitter
- Liquorice – sweet
- Clove – pungent and aromatic
Note: Taste testing should only be performed when the drug is known to be safe; toxic drugs should not be tasted.
D. Size and Shape
The size and shape of the drug are compared with standard descriptions.
Example: Nux vomica seeds are disc-shaped and characteristic in appearance.
E. Surface and Texture
The external surface and texture are examined.
Examples:
- Smooth
- Rough
- Wrinkled
- Hairy
- Fibrous
- Hard
- Soft
F. Fracture
The appearance of a drug after breaking it is examined.
The fracture may be:
- Fibrous
- Short
- Splintery
- Granular
- Uneven
Advantages
- Simple
- Quick
- Inexpensive
- Requires no sophisticated equipment
- Useful for preliminary identification
Limitation
It depends heavily on the experience of the examiner and may not detect closely resembling adulterants.
2. Microscopic Evaluation
Microscopic evaluation involves examination of the drug under a microscope.
It is especially useful for:
- Identification of crude drugs
- Detection of adulteration
- Identification of powdered drugs
- Studying diagnostic cellular structures
Microscopic evaluation is divided into:
A. Qualitative microscopy
B. Quantitative microscopy
2A. Qualitative Microscopy
Qualitative microscopy identifies the characteristic microscopic structures present in a drug.
It answers the question:
“What structures are present?”
Important structures include:
- Epidermal cells
- Stomata
- Trichomes
- Fibres
- Vessels
- Stone cells
- Cork cells
- Calcium oxalate crystals
- Starch grains
- Oil glands
- Secretory cells
- Aleurone grains
Example
In powdered drugs, microscopic examination can help identify characteristic features such as:
Ginger → starch grains and oleoresin cells
Clove → oil cells and characteristic vascular tissues
Senna → characteristic stomata and unicellular/non-glandular trichomes
Section Cutting
For whole or intact plant material, sections may be prepared to study internal structures.
Common sections include:
- Transverse section (T.S.)
- Longitudinal section (L.S.)
- Surface preparation
These help study the arrangement of tissues and cells.
Powder Microscopy
Powdered drugs often cannot be identified properly by external appearance.
Therefore, the powder is examined microscopically for its diagnostic characters.
This is very useful in detecting adulteration of powdered crude drugs.
2B. Quantitative Microscopy
Quantitative microscopy involves the measurement or counting of specific microscopic characters.
It answers:
“How much or how many structures are present?”
It is particularly useful for evaluating leaf drugs.
Important quantitative parameters include:
- Stomatal number
- Stomatal index
- Palisade ratio
- Vein-islet number
- Vein termination number
- Trichome number
1. Stomatal Number
It is the average number of stomata present per unit area of epidermis. It can help in identifying and comparing leaves.
2. Stomatal Index
Stomatal index is the percentage of the total number of epidermal cells that are stomata.
Formula:
Where:
- S = Number of stomata per unit area
- E = Number of ordinary epidermal cells in the same unit area
Important
Stomatal index is generally considered more useful than stomatal number because it is less affected by the degree of leaf expansion.
3. Palisade Ratio ⭐
It is the average number of palisade cells beneath each epidermal cell. It is determined by examining the leaf surface and calculating the average number of palisade cells associated with a given number of epidermal cells. It is useful for the identification of leaf drugs.
4. Vein-Islet Number
It is the average number of vein-islets per square millimetre of leaf surface.
Vein-islet
A vein-islet is the small area of leaf tissue surrounded by the ultimate divisions of the veins.
It is useful in the identification of certain leaf drugs.
5. Vein Termination Number
It is the average number of veinlet terminations per square millimetre of leaf surface. It can also be used as a diagnostic character for leaf identification.
6. Trichome Number
It represents the number of trichomes present per unit area.
Trichomes may be:
- Unicellular
- Multicellular
- Glandular
- Non-glandular
Their number, type and structure can help in drug identification.
3. Physical Evaluation
Physical evaluation involves determining the physical properties of a crude drug.
Important physical parameters include:
- Moisture content
- Ash values
- Extractive values
- Melting point
- Boiling point
- Specific gravity
- Optical rotation
- Refractive index
- Viscosity
- Solubility
- Volatile oil content
A. Moisture Content
Moisture content is important because excessive moisture can promote:
- Growth of microorganisms
- Fungal growth
- Insect infestation
- Enzymatic degradation
- Deterioration of the drug
Therefore, crude drugs should be properly dried and stored.
B. Ash Values ⭐
Ash values indicate the amount of inorganic matter present in a drug.
They are useful for detecting:
- Adulteration
- Contamination
- Excessive soil or sand
- Other inorganic impurities
Types of Ash Values
1. Total Ash: It represents the total amount of inorganic residue remaining after incineration.
It includes:
- Physiological ash
- Non-physiological ash
2. Acid-Insoluble Ash: It represents the portion of ash that is insoluble in dilute hydrochloric acid.
It mainly indicates siliceous matter, such as:
- Sand
- Silica
- Earthy materials
3. Water-Soluble Ash: It is the portion of total ash that is soluble in water.
4. Sulphated Ash: The drug is treated with sulphuric acid before/during ignition.It is used to determine inorganic residue and is particularly useful in certain pharmacopoeial analyses.
4. Extractive Values ⭐
Extractive value indicates the amount of active constituents extracted by a particular solvent from a drug.
It is useful when the drug does not contain a suitable quantity of constituents that can be determined by direct chemical assay.
Important types:
- Alcohol-soluble extractive
- Water-soluble extractive
Importance
Extractive values help to:
- Detect exhausted drugs
- Determine quality
- Detect adulteration
- Estimate soluble constituents
Example
If a drug normally has a certain water-soluble extractive value and the sample has a much lower value, it may indicate poor quality or exhausted material.
5. Chemical Evaluation
Chemical evaluation involves identification and estimation of the chemical constituents present in a drug.
It is used to determine:
- Identity
- Purity
- Quality
- Strength
- Active constituents
Chemical constituents may include:
- Alkaloids
- Glycosides
- Tannins
- Flavonoids
- Saponins
- Steroids
- Terpenoids
- Volatile oils
- Resins
- Carbohydrates
- Proteins
Types of Chemical Evaluation
1. Qualitative Chemical Tests
These tests determine whether a particular constituent is present or absent.
Example:
Tests for:
- Alkaloids
- Tannins
- Glycosides
- Flavonoids
2. Quantitative Chemical Tests
These determine the amount or concentration of a particular active constituent.
For example, estimation of a specific alkaloid, glycoside or volatile oil component.
Chromatographic Methods
Modern chemical evaluation commonly uses:
- TLC – Thin Layer Chromatography
- HPTLC – High-Performance Thin Layer Chromatography
- HPLC – High-Performance Liquid Chromatography
- GC – Gas Chromatography
These techniques are useful for:
- Identification
- Purity testing
- Detection of adulterants
- Fingerprinting
- Quantitative estimation
6. Biological Evaluation
Biological evaluation is the determination of the potency or activity of a drug by using biological methods.
It is also called bioassay when the biological activity is quantitatively measured.
Biological evaluation is particularly important when:
- The active constituent cannot be easily determined chemically.
- Chemical tests do not adequately indicate biological activity.
- The therapeutic activity needs to be measured directly.
Principle
The biological activity of the test sample is compared with that of a standard preparation under controlled conditions.
The response produced by the test sample is measured and compared with the standard.
Example
Biological assays may be used for substances such as:
- Digitalis preparations
- Insulin
- Certain hormones
- Vaccines
- Antibiotics
Types of Biological Evaluation
1. In vivo method
The drug is tested in a living organism.
Examples may include experimental animals where appropriate and ethically approved.
2. In vitro method
The drug is tested outside the living organism using:
- Isolated tissues
- Cells
- Microorganisms
- Enzyme systems
Comparison of Different Evaluation Methods
| Method | Main Purpose | Examples |
| Organoleptic | Examine sensory characters | Colour, odour, taste, shape |
| Microscopic – Qualitative | Identify microscopic structures | Stomata, trichomes, fibres |
| Microscopic – Quantitative | Measure/count microscopic characters | Stomatal index, palisade ratio |
| Physical | Determine physical constants/properties | Ash, moisture, extractive value |
| Chemical | Identify/estimate chemical constituents | Alkaloids, glycosides, HPLC |
| Biological | Determine biological activity/potency | Bioassay, tissue/cell response |
Editorial Note
This article has been carefully researched and written by Deepak Rajput with a focus on accuracy, clarity, and evidence-based healthcare information. Pharmaacademias.com
