تفضل اخوي معلومات بسيطه



Specimen collection and other preanalytical variables

Specimen collection

Examples of biological specimens analyzed in clinical laboratories include whole blood ,serum ,plasma , urine, feces, sweat ,saliva ,cerebrospinal, synovial, amniotic, pleural, pericardial and peritoneal fluids, and various types of solid tissues.

Blood

Blood is the fluid that the heart circulates through the body arteries , capillaries ,and veins. It consists of plasma ,erythrocytes ,leukocytes and platelets. Blood for analysis may be obtained from veins, arteries or capillaries. Venous blood is usually the specimen of choice. And venipuncture is the method used to obtain this specimen. in young children skin puncture frequently is used to obtain what is predominantly capillary blood, arterial puncture is used mainly for blood gas analysis.
Location

The median cubital vein in the antecubital fossa or crook of the elbow is the preferred site for collection of venous blood in the adults because the vein is both large and close to the surface of the skin.
Veins on the back of the hand or at the ankle may be used but these are less preferable and should be avoided in diabetics and other patients with poor circulation. In severely ill patients or those requiring many intravenous injection an alternative blood-drawing site should be selected (selection of a vein for puncture by palpation). An arm containing a cannula or arterio-venous fistula should not be used without consent of the patient physician.

Preparation of the site

The area around the intended puncture site should be cleaned with a prepackaged alcohol swab or gauze pad saturated with 70%isopropanol. Cleaning of the puncture site should be preformed in circular motion and outward from the site. The skin should air-dry. No alcohol should remain on the skin because trace of it may cause hemolysis and invalidate test results. When specimens are to be collected for ethanol determinations the skin should be cleaned with alcohol-free, benzalkonium chloride solution 1:750 .povidone-iodine should be avoided as a cleaning agent because it may interfere with several analytical procedures. Once the skin has been cleaned it should not be touched until after the venipuncture has been completed.

Timing

The time at which a specimen is obtained is important for those blood constituents that undergo marked diurnal variation (as cortisone and iron) and those used to monitor drug therapy (as digoxin and prothrombin time) because the interval after drug administration affects the drug concentration.





Collection with evacuated blood tube

Evacuated blood tubes are usually less expensive and considered more convenient and easier to use than syringes. Many types of evacuated tubes are available for venipuncture collection. They differ by the additive used and the volume of the tubes. Some tubes are siliconized to reduce adhesion of clots to walls or stoppers and decrease risk of hemolysis. The silicone-coated wall also may activate and accelerate the clotting mechanism. blood collected into a tube containing one additive should never be transferred into another tube because the first additive may interfere with tests for which a different additive is specified .thrombin is added to some tubes to accelerate clotting.
After the skin is cleaned the needle should be aligned with the intended vein and pushed into the vein at an angle approximately 15 degree to the skin. Once the needle is in place the tube should be pressed forward into the holder to puncture the stopper and release the vacuum. when blood begins to flow into the tube, the tourniquet should be released without movement of the needle. The tube is filled until the vacuum is exhausted, and the tube then is withdrawn from the holder and replaced with another tube. Other tubes may be filled if required Through use of the same technique and with the holder in the place, a shut-off valve consisting of rubber tubing often is used to prevent blood spillage .
Blood collection with the syringe

Syringes are used on patients with difficult veins. The needle is placed firmly over the nozzle of the syringe and the needle cover removed. The syringe and the needle should be aligned with intended vein and the needle pushed into the vein at an angle approximately 15 degrees to the skin. When the initial resistance of the vein wall is over come as it is pierced, forward pressure on the syringe is eased and the blood is withdrawn through gentle pulling back of the syringe plunger. If a second syringe is needed a gauze pad may be placed under the hub of the needle to absorb spill. After removal of the needle from the syringe drawn blood should be transferred quickly by gentle ejection into prepared tubes.
The tubes then should be capped, if they contain additives or anticoagulants, they should be mixed gently (5-10 inversions). Vigorous suction on a syringe during collection or transfer from the syringe to the receiving vessel should be avoided because it may be cause hemolysis. When blood collection is complete, the needle is withdrawn and the patient asked to hold a dry gauze pad over the puncture site, with the arm raised to lessen the leakage. The pad is held in place by a bandage which is removed after 15 minutes. The used needle from the syringe or collection tube should be discarded into a sharps container.

Venipuncture in children

The techniques for venipuncture in children and adults are similar. However, children are likely to make unexpected movements. either a syringe or evacuated blood tube system may be used to collect specimens, a syringe should be either the tuberculin type or a 3-mL-capacity syringe, except when a large volume of blood is required for analysis. A 21-23 gauge needle or 20-23 gauge butterfly needle with attached tubing is appropriate to collect specimen.

Arterial puncture

Arterial puncture requires considerable skill and usually is performed only by physicians or specially trained technicians or nurses. The preferred sites of arterial puncture in order as follows: 1-radial artery at the wrist
2-the brachial artery in the elbow
3-the femoral artery in the groin because the femoral artery tends to leak a greater amount of blood especially in elderly patients sites in the arm are used most often. In the neonate an indwelling catheter in the umbilical artery is best to obtain specimens for blood gas analysis. In the older child or adult in whom an arterial puncture is not possible.
Capillary puncture may be performed to obtain arterial capillary blood, such a specimen yields acceptable values for ph and pco2 but not always for po2.in older child or adult the preferred puncture site is the earlobe in young child or infant it is the heal. Blood specimens are particularly inappropriate when blood circulation is poor and thus should be avoided when patient demonstrates reduced cardiac output, hypotension or vasoconstriction.

Anticoagulants and preservatives for blood

To prepared a whole blood or plasma sample an anticoagulant must be added to the specimen during the collection procedure. a number of anticoagulants are used including heparin, ethylendiaminetetraacetic acid (EDTA), sodium fluoride, citrate, oxalate and sodium iodoacetate. Triglyceride, albumin, alkaline phosphatase, aspartate amino transferase, bicarbonate, creatine kinase, glucose, phosphate, potassium, sodium, urea, uric acid concentration is higher in serum than in plasma but calcium, chloride, lactate dehydrogenase, total protein concentration is greater in plasma than in serum. bilirubin, cholesterol, creatinine concentrationhas no difference between serum and plasma values.
Glucose, potassium value is higher in capillary than in venous blood, Bilirubin, calcium, chloride, sodium, total protein value is greater in venous than capillary blood, phosphate, urea value is same.


Heparin

Heparin is the most widely used anticoagulant and causes the least interference with the tests. It is a mucoitin polysulfuric acid and is available as sodium, potassium, lithium and ammonium salts. its action through acceleration of antithrombin3. most blood tubes are prepared with approximately 0.2 mg heparin for each milliliter of blood collected. The heparin usually is present as a dry powder that dissolves rapidly.

Ethylenediaminetetraacatic acid(EDTA)

EDTA is a chelating agent that forms a complex with the calcium necessary for coagulation. It is used as the di-sodium, di-potassium or tri-potassium salts, the latter 2 being more soluble. it is effective as a final concentration of 1 -2 mg/mL of blood, higher concentrations hypertonically shrink the red cells. EDTA inhibits alkaline phosphatase , creatine kinase and leucine aminopeptidase activities. its chelation of calcium makes EDTA unstable for specimens of calcium and iron analyses using photometric technique.

Sodium fluoride

Sodium fluoride is a weak anticoagulant but often is added as an antiglycolytic agent to preserve blood glucose. it is effective at a concentration of approximately 2mg/mLblood as a preservative with another anticoagulant such as potassium oxalate. Most specimens are preserved at 25 ˚C for 24 hours or at 4 ˚C for 48 hours. Without antiglycolytic agent the blood glucose concentration decrease approximately 10mg/dL per hour at 25 ˚C. The rate of decrease is faster in newborns because of the increased metabolic activity of their erythrocytes, and in leukemic patients because of the high metabolic activity of WBCs. Sodium fluoride is poorly soluble and blood, Must be mixed thoroughly. When sodium fluoride is used alone anticoagulation3 -5 times greater concentrations than the usual 2mg/mL are required. This high concentration and inhibition of glycolytic cycle are likely to cause fluid shifts and change the concentration of some analytes. Fluoride is also a potent inhibitor of many serum enzymes and in high concentrations also affects urease enzyme that used to measure urea nitrogen.

Citrate

Sodium citrate solution at a concentration of3.2 -3.8 g/dL in a ratio of 1 part to 9 parts of blood is used widely for coagulation studies because the effect is reversible easily by the addition of Ca††. It also inhibits aminotransferase and alkaline phosphatase but stimulates acid phosphatase .

Oxalates

The sodium, potassium, ammonium and lithium salts of oxalic acid inhibit blood coagulation by forming rather insoluble complex with calcium ions. Potassium oxalate at a concentration of approximately 1-2 mg/mL of blood is the most widely used oxalate. At concentration of greater than 3mg oxalate/ml blood hemolysis is likely to occur. combined ammonium/potassium oxalate does not cause shrinkage of erythrocytes. However other oxalates cause shrinkage leading to reduction in hematocrit may be as much as 10% causing a reduction of 5% in the concentration of plasma constituents.



Sodium iodoacetate

At a concentration of 2 g/L sodium iodoacetate is an effective antiglycolytic agent. Because it has no effect on urease it has been used when glucose and urea tests are preformed on a single specimen. Sodium iodoacetate inhibits creatine kinase but appears to have no significant effects on other clinical tests.


Urine

Urine is the fluid excreted by the kidneys, passed through the ureters, stored in the bladder and discharged through the urethra. it has an amber color and a peculiar odor and is slightly acidic. 1000-2000mLof urine is excreted every 24 hours. its specific gravity varies from 1005-1030.

Types of urine specimen

1-Randum specimen: it is collected at an unspecified time, often for drugs of abuse is to be performed.
2-First-morning or 8-hours specimen: it is collected after an individual arises from a night sleep. It is the choice for microscopic examination as well as for detection of abnormal amount of protein or unusual compounds as chorionic gonadotropin .
3-Timed specimen: it is collected over a predetermined interval of time such as 1, 4 or 24 hours. A patient should be instructed about diet or drug ingestion before beginning.
4-24hours specimen:it is collected such for measuring of catecholamines,17-hydroxysteriod
5-Clean-catch specimen: while the patient urinates a midportion is collected into a sterile, inert plastic disposable urine container(50-100 mL) without contamination of the container. neither the urine stream nor the specimen container should touch the perineum. This specimen is used for bacterial culture.
6-Catheter specimen: a catheter is inserted into the bladder , a urine specimen then is collected from outflow end of the catheter. this is used for microbiological examination in critically ill patients or in patients with urinary tract obstruction.
7-suprapubic specimen: it is collected to obtain a specimen directly from the blader which is
Uncontaminated by urethral bacteria. This type of collection is useful in infants. A needle is inserted through the thoroughly the cleaned suprapubic skin and abdominal wall into the distended bladder, a specimen of urine is obtained via aspiration.
8-Urine collection from children: from older children is done as in adults through assistance of a parent, but newborn and pediatric urine specimen collection bags with skin adhesive are available for such specimens. If a urine specimen cannot be transported and analyzed immediately, it should be refrigerated at 2 to 8 ˚C or frozen at -24 to -16˚C after collection with addition of preservatives such as boric acid.


Feces

Small aliquots of feces frequently are analyzed to detect the presence of hidden blood so called occult blood, which is recognized as one of the most effective clues to the presence of a bleeding ulcer or malignant disease in the gastrointestinal tract. Measurement of the fecal nitrogen and fat in 72-hours specimen is used to assess the severity of malabsorption, measurement of fecal porphyrins occasionally is required to characterize the type of porphyria.

Sweat

The analysis of the sweat for increased electrolyte concentration is used to confirm the diagnosis of cystic fibrosis.

Saliva

Saliva is the clear, alkaline, viscous fluid secreted from the parotid, sub- maxillary, sublingual and small mucous glands of the mouth. Approximately 0.8 -1.2 L of saliva is produced daily. Constituents such as viruses, bacteria, proteins, hormone, therapeutic drugs and drugs of abuse have been measured in saliva. in one such kit saliva collection the patient must chew an absorption roll of dental cotton for several minutes. After being soaked with saliva the roll is placed into a plastic tube and the saliva collected in the bottom of the tube by centrifugation. another device uses a vacuum-type aspirator similar in concept to a dental aspirator.

Amniotic fluid

During pregnancy amniotic fluid lies within the amniotic cavity and is produced by the amnion during embryonic period and later by the kidney and lungs. The amount in a full-term patient normally varies from 500 -1500mL. The technique used to obtain a specimen of amniotic fluid is called amniocentesis

Pleural, pericardial and peritoneal fluids

Serous fluids are found in the pleural, pericardial and peritoneal cavities where they lubricate the opposing parietal and visceral membrane surfaces. Inflammation or infection affecting the cavities causes fluid to accumulate. The process involving excessive accumulation of free fluid in the abdomen is known as ascites. The fluid may be removed to determine whether it is an effusion or exudate. Depending to the cavity sampled the collection procedure is called thoracocentesis (chest), pericardiocentesis(heart) or paracentesis (abdomen). The skin over the intended puncture site should be cleaned with 70 %isopropanol and allowed to air dry, a spinal needle then is inserted into the body cavity through a small bleb in the skin raised by injection of a local anesthetic. A syringe is used to withdraw fluid which is transferred to appropriate tubes for analysis.

Solid tissue

Many types of solid tissue are sent to the laboratory for analysis, malignant tissue from tumors and hair are examples. Malignant tissue as breast tissue during surgery at least 0.5 to 1g of tissue should be removed and trimmed of fat and non tumor material. The tissue should be frozen rapidly, preferably in liquid nitrogen or mixture of dry ice and alcohol. The time between collection and freezing should be less than 20minutes. Hair is used as a sample for trace and heavy-metal analysis. The analysis of hair for the presence of drugs has increased because more than 50 pharmaceuticals or drugs of abuse have been detected in hair after oral or parenteral use.

Handling of specimen for testing

Currently, bar coding is the identification technology of choice. This identifying link must be maintained throughout the collection process, transport of the specimen to the laboratory, subsequent analysis and preparation of report.
the specimen must be treated properly from the time of its collection during its transport to the laboratory, to the point at which it is analyzed. for some testes such as ammonia and blood gas determination specimen must be preserved at -4˚Cfrom the time the blood is drawn until the specimen is analyzed or the serum or plasma is separated from the cells.
For all test constituents that are thermo labile, serum and plasma should be separated from cells in a refrigerated centrifuge. Specimens for bilirubin or carotene must be protected from both daylight and fluorescent light to avoid photo degradation.
If the specimen cannot be analyzed at once, the separated serum usually should be stored in a capped tube at 4 ˚C until the time of analysis. The time required to transport a specimen from its time of collection until it reaches the laboratory varies from a few minutes to as long as 72 hours. The container or tube used to hold a specimen should be constructed so that the contents do not escape if the container is exposed to extremes of heat, cold or sunlight.

Excellent
Excellent

معلومات اكثر من رائعه
الله يعطيك العافيه يارب:sm174: