Pre-analytical variables : Various types of Sample collection procedure in clinical laboratory (Part 2)

 Urine Collection

• Untimed, random specimens
• Pre-determined interval of time – 1, 4, or 24 hours\
• Clean, morning, fasting specimen – Preferred for most tests.
• Clean, timed specimen – Specific times of the day.
• Mid-stream specimen – Bladder disorders
• Double-voided specimen – GTT
• Metabolic disorders – Acute phase
• Catheter specimen – Microbiological examination in critically ill
• Suprapubic tap – After cleaning skin, 22G spinal needle introduced into a small wheal with LA, bladder penetrated and urine withdrawn.
• Genitalia cleaned to avoid chances of contamination, especially for urine culture.

Timed Urine Specimens

• Should be long enough to minimize influence of short-term biological variations.
• Bladder must be emptied at the time collection is to begin, and this must be discarded.
• Afterwards, all urine must be collected.
• Precautions must be taken to prevent bowel contamination.
• 2 containers, one for collecting urine and the other one, for storing collected urine are advisable.
• Larger container for storing should be kept at 4°C.
• Written instruction regarding diet, drugs etc. would be very useful and must be encouraged.
• For example, VMA and HIAA, avocados, bananas, plums, walnuts,
• pine apples, eggplant, acetaminophen, cough syrups to be avoided.

 Urine should not be collected at the same time for 2 or more tests requiring different preservatives.

• Aliquots should not be removed till the entire timed specimen is collected, even if volume is measured.
• Metabolic bed – For collection to timed specimens from infants is available.
• Infants lies on a fine screen above a funnel shaped base containing a drain under which a container is placed to receive urine.
• Screen retains fecal matter. However, urine is likely to be contaminated.
• Need for timed specimen for infants is less.
• Sterile urine may be obtained by suprapubic tap.

Urine Preservatives

• Glacial acetic acid – Aldosterone, catecholamines, cortisol, estrogens, 17 ketosteroids, metanephrine.
• Boric acid – Homovanillic acid, hCG, VMA
• Concentrated HCl – Amino acids, calcium, copper, catecholamines, 5HIAA, VMA, hydroxyproline, magnesium, mercury, metanephrines, oxalate, nitrogen
• Mild base – Porphyrins, amino levulinic acid
• Chloroform – Organic acids
• Nitric acid - Mercury

Feces

• Used for occult blood – Bleeding ulcer or GIT malignancy
• Used for periodic health checks.
• Done on aliquots of stools, and not obtained from per rectal examination.
• Even stools from toilet bowl can be used.
• Applied to reagent impregnated slides.
• Sent to laboratory for evaluation.
• Tryptic activity – In children for detecting cystic fibrosis.
• Can be recovered from diapers.
• Malabsorption – Fecal nitrogen and fat assayed in 72-hr specimen.
• Fecal porphyrins – For porphyrias.
• No preservative needed, but feces is refrigerated throughout.
• Container and feces are weighed, mass of excreted feces is calculated.
• Specimen homogenized, aliquoted and amount of fat or nitrogen per day calculated.
• Metabolic balance studies – Collection over 72 hours. Usually done for metal evaluation (calcium). Diet precautions are important.

Spinal Fluid

• Obtained from lumbar region, or cervical region, cistern or ventricle of brain.
• Indications - Cerebro-vascular accident, meningitis, demyelinating disease, menigneal involvement in malignant disease.
• Must be performed only by physician.
• Clean the skin of the lumbar region below the termination of spinal cord, where cauda equina passes through the spinal canal.
• Make a small bleb in the skin over the space between the third and fourth, or fourth and fifth lumbar vertebra with 2% procaine.
• Introduce spinal needle (22G, 9 cm) through the bleb.
• Pressure monitored, and 3-4 mL fluid collected.
• Sterile tube should be used.
• First tube may be contaminated, hence second and third tubes used for culture and microscopy.
• Same procedure for infants, but fluid withdrawn should be minimum needed for test.
• Up to 20 mL can be safely drawn from adults.
• Rapid processing of specimens is a must and no preservatives are needed.
• Simultaneous blood specimen should be taken (e.g. for glucose).

Pleural, Pericardial and Ascitic Fluids

• Small amount of serous fluid to lubricate is normally present.
• Procedure termed paracentesis, for pleural cavity, thoracocentesis, pericardial cavity, pericardiocentesis.
• Should be done by physician.
• Complications – Rarely seen, blood stained fluid, adhesions in intestine can lead to perforations, pneumothorax and bronchopleural fistula (thoracocentesis).

Saliva

•  Limited applications like measurement of blood group substances and drugs.
• Individual is asked to rinse mouth, chew an inert material like rubber for a fixed time (30 sec to a few minutes).
• Initial saliva sample discarded and thereafter collected in small glass bottles.

Swabs

• Used commonly for microbiological testing.
• Also used for molecular genetics testing.
• Sterile dacron or rayon swab with plastic shaft is used.
• After collection, swab is stored in air-tight plastic container or immersed in liquid, such as phosphate buffered saline (PBS) or viral transport medium.

Amniotic Fluid (Amniocentesis)

• For prenatal diagnosis of congenital disorders, to assess fetal maturity, look for Rh-isoimmunization or intrauterine infection.
• Best performed with ultrasound guidance.
• Skin is cleaned and anesthetized, 10 ml fluid aspirated into spinal needle.
• Sterile containers used.
• Complications include bloody tap from uterine wall, placenta or fetus.
• Specimen may be wrapped in aluminium foil.
• For L/S ratio, container is immersed in ice.
• For spectrophotometric analysis, transferred in brown bottle to prevent degradation of bilirubin.

Solid tissues

• Analyzed mainly for malignant tissues, e.g. breast for biopsy.
• 0.5 – 1.0 g tissue removed and trimmed of fat and non-tumor material.
• Tissue quickly frozen within 20 min.
• A histological specimen should always be examined at the time of analysis of the specimen to confirm malignant nature.
• For trace elements and toxicology, plastics should be free of contamination.
• Synovial fluid (Arthrocentesis) – Arthritis, to diferentiate non-inflammatory effusions from inflammatory fluids.
• Hair and finger nails – For trace elements or drugs. Methods are poorly standardized.
• Sweat – Chloride for cystic fibrosis.

(Source: Supplementary information , Textbook of Biochemistry for Medical Students, 7th edn.)