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Specimen collection, handling, and processing remain ones of the primary areas of pre-analytic error. Careful attention to each phase is necessary to ensure proper subsequent testing and reporting of meaningful results, along with the documentation of the resolution of any errors.
When hospital personnel – rather than laboratory technicians- who may be collecting samples (e.g. nurses, physicians) may not have the benefit of such training, and improper collection of blood samples as well as an increase in errors may result of lack of training.
1- Hemolysis – a mechanical destruction of RBCs that result in Hemoglobin release. Specimens should be centrifuged for approximately10 min at an RCF of1000 –2000 g to avoid this. Hemolysis may result in such tested analyte of request to be elevated level or decreased.
2- Improper patient identificationor improper tubes labeling at sample collection process can result in sample discarding. The main source of Pre-analytic errors can be formed from incorrect matching the blood collection tubes with the analyte request and patient identification labels.
3- Intravenous contaminationshould be considered and it will be formed if a large increase occurs in the substances being infused, such as Glucose, Potassium, Sodium, and Chloride, with a decrease of other analytes such as Urea and Creatinine.
4- In serum or plasma samples, the presence of Icterus (increased bilirubin pigment) or turbidity often associated with Lipemia (increased lipids), will lead to discard the sample and inquiry for a new sample.
5- Required time for analysis after sample drawing:
- Samples should be analyzed within4 hrs in order to minimize the effects of evaporation, and should be properly capped and kept away from areas of rapid airflow, light, and heat.
- Proper storage, mostly by refrigeration at4 C for8 hrs should be performed if testing is to occur after that time.
- Many analyte are stable at this temp, with the exception of Alkaline Phosphatase (increase) and Lactate dehydrogenase (decreases as a result of temperature).
- Repeated cycles of freezing and thawing, like those that occur in so-called frost –free freezers, should be avoided.
6- Fasting samples:
- Most samples are drawn on patients who are fasting (usually overnight for at least8 hrs). Because overnight and fasting are relative terms, however, the length of time and what are consumed during that time should be determined before sample procurement for those tests most affected by diet or fasting.
- For Bilirubin assay, a fasting serum specimen is preferred to avoid hemolyzed or lipemic samples in nature. Before testing, serum should be stored in the dark and measured as soon as possible (within2-3 hrs) after collection. Hemolyzed specimen will cause a decrease in serum bilirubin,
7- The use of evacuated tubes, which may incorporate plastic, gel-like material – that serves as a barrier between the cells and the plasma or serum and seals these compartments from one another during centrifugation – can interfere with certain analytes by its gel, notably trace metals, and drugs such as Tricyclic Antidepressants.
8- Alcohol Disinfection- When it is required to draw blood alcohol levels to measure it, isopropyl alcohol is not recommended as proper antiseptic for drawing site disinfection.
9- Prolonged tourniquet applicationcauses a stasis of blood flow and an increase in hemoconcentration and anything bound to proteins or the cell, and false increase in ionized calcium.
10- For ionized calcium measurements, samples must be collected anaerobically, because loss of CO2 will increase pH, and heparin anticoagulant must be avoided because it binds with calcium and lower ionized calcium conc.
11- Presence of interferences:
n It is important to ascertain that, if any, medications that patient is taking that may interfere with the test.
n Smoking may cause an increase in glucose as a result of the action of nicotine; growth hormone; cortisol; cholesterol; triglyceride and urea.
n High amounts or chronic consumption of alcohol causes hypoglycemia, increased triglycerides and an increase in the enzymes gamma – glutamyltransferase and other liver function tests.
n Intramuscular injections increase the enzyme CK and the skeletal muscle fraction of Lactate Dehydrogenase.
n Opiates, such as morphine or Meperidine, cause increases in liver and pancreatic enzymes, and Oral Contraceptives may affect many analytic results.
n Many drugs may affect blood tests; Diuretics can cause Potassium decrease and Hyponatremia; Thiazide-type medications can cause hyperglycemia and perenal azotemia secondary to the decrease in blood volume.
12- For Potassium measurements:
n Having the patients open and close their fist during phlebotomy is of no value and may cause an increase in Potassium (K+) and, therefore, should be avoided.
n At serum samples, significant elevation of platelets count may result in the release of potassium during clotting from rapture of these cells, causing a spurious hyperkalemia. So, serum should be avoided and plasma samples are preferred.
n Infants and children have smaller veins than adults; to ensure that small veins do not collapse, narrow –gauge needles are generally used for venipuncture. Smaller needles increase the risk of hemolysis and hyperkalemia.
13- For Phosphate measurement, circulatory phosphate levels are subject to circadian rhythm, with highest levels in late morning and lowest in the evening.
14- For Ammonia sample:
= Whole blood ammonia conc. Rises rapidly following specimen collection because of in vitro amino acid deamination.
= Venous blood should be obtained without trauma and placed on ice immediately.
= Erythrocytes contain2-3 times as much ammonia as plasma, so as hemolysis should be avoided.
= Precautions must be taken to minimize contamination in the laboratory in which the assay is performed to improve assay accuracy, and sources of contamination include tobacco smoke, urine, and ammonia in detergents, glassware, reagents (ammonium salts, asparaginase, barbiturates, diuretics, ethanol, narcotic analgesics and Diphenylhydramine) and water.
15- For blood gas measurements:
= when collection is painful and result in patient hyperventilation, PCO2 will decrease and pH will increase.
Sources of errors in collection and handling of blood gas specimens include the collection device, from heparin conc., speed of syringe filling, maintenance of anaerobic environment, mixing of sample to ensure dissolution and distribution of heparin anticoagulant, and transport in ice and storage time before analysis.
Thanks
When is Haemolysis sample or fatty in serum
if the sample is coagulated in the tube is another reason to discard it
