Tuesday, August 5, 2014

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tap – After cleaning skin, 22G spinal needle introduced into a small wheal with LA, bladder penetrated and urine withdrawn.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


  • 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).


  •  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.


  • 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.)
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