A biological specimen is material collected from the body for diagnostic testing, analysis, or medical research. These samples are fundamental to healthcare, providing a chemical and cellular snapshot of a person’s internal state. The collection process must be highly standardized to ensure the integrity of the material, as contamination or degradation compromises the accuracy of test results. The choice of specimen type depends entirely upon the specific diagnostic question and the biological markers being sought.
Specimens Collected Through Excretion or Surface Swabbing
This category includes specimens obtained non-invasively or with minimal invasion, often derived from natural bodily outputs or external surfaces. Urine is the most common example, used for comprehensive urinalysis to assess kidney function, check for urinary tract infections, and screen for metabolic disorders like diabetes. Urine is also a standard sample for drug screenings and for detecting human chorionic gonadotropin (hCG) to confirm pregnancy.
Feces provides insight into the health of the gastrointestinal tract. Analysis focuses on detecting pathogens such as bacteria, viruses, or parasites that cause infectious gastroenteritis. Stool tests are also used for screening occult blood, which can indicate internal bleeding from conditions like inflammatory bowel disease or colorectal cancer.
Surface swabbing collects material from mucous membranes to diagnose localized infections. Nasal and nasopharyngeal swabs are standard for testing respiratory viruses, including influenza, respiratory syncytial virus (RSV), and SARS-CoV-2. Throat swabs are collected to isolate bacterial organisms, such as Streptococcus, which causes strep throat.
Saliva and sputum provide different diagnostic information. Saliva is increasingly used for non-invasive testing, valuable for analyzing the biologically active fraction of steroid hormones like cortisol and testosterone. Sputum is material coughed up from the lower respiratory tract, necessary for diagnosing deep-seated respiratory infections like pneumonia or tuberculosis, as it contains pathogens from the lungs.
Blood and Circulatory Components
Blood is the most analyzed biological specimen because it circulates through every organ system, carrying cells, nutrients, hormones, and waste products. The sample is typically obtained via venipuncture, and the collection tube determines the component analyzed. Whole blood contains all cellular components (red blood cells, white blood cells, and platelets) and is used primarily for a Complete Blood Count (CBC) and genetic analysis.
The two main liquid fractions derived from whole blood are plasma and serum, distinguished by whether the blood is allowed to clot during processing. To obtain plasma, whole blood is collected with an anticoagulant, which prevents clotting. Plasma retains all clotting factors, including fibrinogen, making it the required specimen for coagulation studies that assess bleeding disorders or monitor blood-thinning medications.
Conversely, serum is the liquid portion remaining after whole blood is allowed to clot and the clot is removed by centrifugation. Serum is preferred for chemistry and immunology tests because the clotting process removes fibrinogen and other clotting factors. Specialized collection tubes contain different anticoagulants, which must be selected carefully. For example, ethylenediaminetetraacetic acid (EDTA) prevents clotting and is preferred for hematology because it preserves cell morphology. Heparin, another common anticoagulant, is often used for general chemistry panels.
Solid and Invasive Tissue Samples
Solid tissue samples are collected when a disease process is suspected in a specific organ and a cellular-level diagnosis is required, often involving an invasive procedure. A biopsy is the removal of tissue or cells for microscopic examination by a pathologist. This procedure is the definitive method for diagnosing most cancers, chronic inflammatory, or autoimmune diseases.
Biopsy methods vary widely depending on the organ and target depth.
Biopsy Methods
- Fine-needle aspiration (FNA) extracts fluid and cells.
- Core needle biopsies remove a small cylinder of tissue.
- Surgical excisions remove larger sections of tissue or an entire lesion to capture the complete extent of a disease.
Bone marrow aspiration and biopsy are typically performed simultaneously to examine the body’s blood-forming factory, collected from the hip bone. The aspiration collects the liquid cellular component, while the biopsy retrieves a solid core of bone and marrow architecture. These combined samples are essential for diagnosing blood cancers like leukemia and lymphoma, or for investigating unexplained anemia and other blood cell disorders.
All solid tissue must be immediately placed in a fixative, such as 10% neutral buffered formalin, to stabilize the proteins and prevent degradation. The fixed tissue is then processed and prepared for microscopic viewing.
Specialized Internal Body Fluids
Certain biological fluids are contained within closed body compartments and require specialized invasive procedures for collection. Cerebrospinal fluid (CSF) is obtained through a lumbar puncture (spinal tap), where a needle is inserted into the lower back to sample the fluid surrounding the brain and spinal cord. Analyzing CSF helps diagnose serious central nervous system conditions, including infectious meningitis, inflammatory disorders like multiple sclerosis, or cancers that have spread to the central nervous system.
Synovial fluid, which lubricates joints, is collected through arthrocentesis (joint aspiration). Analysis of this fluid diagnoses the cause of acute joint pain and inflammation. Microscopic examination of synovial fluid identifies the presence of crystals that confirm a diagnosis of gout or pseudogout.
Amniotic fluid surrounds a developing fetus and is collected via amniocentesis, usually performed after the 15th week of pregnancy. The fluid contains fetal cells analyzed to diagnose chromosomal abnormalities and genetic conditions, such as Down syndrome or spina bifida. These specialized collections carry a recognized risk of complications, such as a low chance of miscarriage in the case of amniocentesis, and are therefore reserved for targeted diagnostic needs.