Factors affecting lab results

Many factors associated with specimen collection can influence the validity and

interpretation.

These include the collection of specimen from the wrong patient, incorrect identification of specimens after collection and subsequent handling prior to analysis.

FACTORS TO CONSIDER BEFORE COLLECTING SPECIMEN

These include: DietDiet Drugs & current medicationDrugs & current medication Time of dayTime of day

Diet: Some chemical investigations are liable to interference from dietary constituents e.g. a patient on whom Calcium analysis is to be performed should not take dairy products.

Subjects for GTT should be on their normal carbohydrate diets of about 150g of carbohydrate/dayfor at least 3 days prior to the test t. Patients for lipid analysis should be on their normal lipid intake for atleast a week prior to the test.

Drugs/Current Medication:

Many drugs are known to affect chemical/Laboratory

determinations, e.g. Oral Contraceptives, coughmixtures, etc.

Patients on cephalosporins, vitamin C or ascorbicacid are most likely to present with false

glycosuria ifthe method for determining urine glucose is theBenedicts test.

It is important that doctors indicate or discuss details

of the current medication of patients with the laboratory staff.

Time of Day:

Several blood constituents show variation

with time of day, e.g. corticosteroids, iron. For

such blood constituents, it’s very imperative

that the time at which the specimen was collected be indicated.

FACTORS TO CONSIDER AT THE TIME OF

COLLECTING SPECIMEN

PostureVenous stasisThe site of venous punctureHaemolysis

The posture of the patient:

Several proteins and protein-bound blood constituents show significant differences

in concentration between samples collected from upright position and those fromrecumbent (leaning) individuals, e.g.

Albumin, calcium, cholesterol, cortisol and proteinbound iodine.

ctd

Example:

When the posture is changed from lying to standing,

there may be an increase of as much as 13% in the

concentration of non-diffusible components of blood

within 15 minutes due to redistribution of the

extracellular fluid. It’s very important therefore for

every health institution to standardize its procedure

for obtaining blood specimen..

The combined effect of raised intravenous pressure

with its associated local anoxia from sustained

occlusion results in the passage of water and small

molecules from the lumen of a vein into the

surrounding extracellular fluid.ctdctd

Venous stasis:

Every effort must be made at avoiding prolonged stasis during venepuncture since it can markedly raise the concentration of plasma proteins, haemoglobin, hormones, calcium and lipids.

Intracellular constituents such as potassium and

lactic acid leak into the plasma causing falsely

elevated results. The best procedure is to remove

the tourniquet soon after puncturing the vein.

The site of venepuncture:

Under no circumstance should blood be drawn from an arm receiving an intravenous infusion since the fluid may not have mixed with the whole blood Vol.

Blood from the opposite arm usually provides valid

results. If there are no available veins on the arm, then

a femoral blood should be taken. But if this is not

possible, then the specimen should be obtained from a

site distant from the infusion site or preferably about

10-20ml of blood should be aspirated through the

needle (if patient is very sick, don’t).

Haemolysis: This must be avoided at all costs because it invalidates

certain determinations owing to the release of erythrocytic

contents, e.g. Potassium, lactate dehydrogenase, acid

phosphatase or through colour changes which interfere with

photometric determination using shorter wavelengths of the

visible spectrum (400-500nm).

Haemoglobin also interferes with chemical reactions such as

the diacetylation of bilirubin. To minimize haemolysis, the

specimen should be collected with only moderate suction.

The plunger of the syringe should not be drawn too fast and

there should be an easy flow of blood

The needle should be removed from the syringe

before the specimen is expelled gently into an

appropriate container leaving behind any froth that

may be in the syringe. Tubes into which the blood is

expelled should be cleaned and dry.

Any anticoagulant in the tube should be mixed with

the blood by gentle rotation. If the blood is taken on

the ward, and allowed to stand for more than an hour

or two, the effect on the plasma constituents is

synonymous to haemolysis.

PRESERVATION OF SPECIMEN IN TRANSIT

Specimen must be handled properly from the time the specimen has been separated until it is analysed.

For some tests. e.g. blood gas determination e.g. PCO2 PO2, and blood pH, the sample must be kept at 40C from the time the sample is drawn till serum or plasma is separated from cells.

Transfer of specimen to the lab must be done by placing the specimen in a container of ice/ice cold water.

For hormonal assays e.g. gastrin, renin & PTH serum must and should be separated from the cells in a refrigerated centrifuge.

Specimen for bilirubin and carotene must be protected from both sunlight and fluorescent light to avoid photo degradation

SEPARATION AND STORAGE OF SPECIMEN

Centrifuge with stoppers in place.Centrifuge with stoppers in place.

Closure of the tubes reduces evaporationClosure of the tubes reduces evaporation

Stoppers also prevent aerosol formation Stoppers also prevent aerosol formation from infectious particlesfrom infectious particles

Maintain anaerobic conditions required for Maintain anaerobic conditions required for certain determinations such as COcertain determinations such as CO2 & Ca& Ca22++

LONG-TERM BIOLOGICAL CHANGES: Biovariability

the influence of race Gender Age Neonates Childhood- puberty The young adult The elderly adult

The influence of race The influence of race

There is a genetic difference in the plasma There is a genetic difference in the plasma concentration of cholesterol, glucose, urea, concentration of cholesterol, glucose, urea, urate, and bilirubin. urate, and bilirubin.

Blacks are known to have a higher serum Blacks are known to have a higher serum protein than whites. This is probably due to the protein than whites. This is probably due to the higher level of gamma globulin in blacks. The higher level of gamma globulin in blacks. The IgG is often 40% higher and IgA 20% higher in IgG is often 40% higher and IgA 20% higher in blacks as compared to whites. blacks as compared to whites.

The serum albumin of Blacks is typically lower. The serum albumin of Blacks is typically lower.

The activities of creatinine kinase and lactate The activities of creatinine kinase and lactate dehydrogenase are usually higher in blacks dehydrogenase are usually higher in blacks than whites due to higher skeletal muscle mass. than whites due to higher skeletal muscle mass.

Blacks generally have serum alkaline Blacks generally have serum alkaline phosphate activity at puberty than white phosphate activity at puberty than white children. After 40 years serum cholesterol and children. After 40 years serum cholesterol and triglycerides are higher in whites than in blacks. triglycerides are higher in whites than in blacks.

Glucose tolerance is less in Blacks, American-Glucose tolerance is less in Blacks, American-Indians and Eskimos than in comparable age Indians and Eskimos than in comparable age and sex matched Whites.and sex matched Whites.

GenderGender Until puberty there are a few differences in lab Until puberty there are a few differences in lab

data between boys and girls. data between boys and girls.

After puberty, the serum activity of: After puberty, the serum activity of: a] alkaline phosphatase, a] alkaline phosphatase, b] aminotranferase, b] aminotranferase, c] creatine kinase c] creatine kinase d] aldolase are higher in men than women. d] aldolase are higher in men than women.

After menopause, the activity of alkaline After menopause, the activity of alkaline phosphatase increases until it is higher in phosphatase increases until it is higher in women than men. women than men.

The concentration of albumin, calcium and The concentration of albumin, calcium and Mg2+ are higher in men than in women. But Mg2+ are higher in men than in women. But the concentration gamma globulin is less in the concentration gamma globulin is less in men than in women Hb and serum bilirubin are men than in women Hb and serum bilirubin are less in females,. less in females,.

Serum iron is low during a woman’s Serum iron is low during a woman’s reproductive life and her plasma ferritin is reproductive life and her plasma ferritin is usually one –third that of males. Cholesterol is usually one –third that of males. Cholesterol is typically higher in men than women. typically higher in men than women.

Plasma concentration of amino acids creatinine, Plasma concentration of amino acids creatinine, urea, uric acid, are higher in men than women. urea, uric acid, are higher in men than women.

Hyperuricaemic diseases like gout affect more Hyperuricaemic diseases like gout affect more men than women.men than women.

Age Age : : Generally individuals can be divided into the ff Generally individuals can be divided into the ff

groups: groups: a) Neonates, a) Neonates, b) Childhood-Puberty, b) Childhood-Puberty, c) The Young Adult & c) The Young Adult & d) Elderly Adultd) Elderly Adult

NeonatesNeonates:: The serum activity of several The serum activity of several enzymes including Creatine Kinase (CK), enzymes including Creatine Kinase (CK), Gammaglutamyl transferase (GGT) and Gammaglutamyl transferase (GGT) and Aspartate transaminase are high at birth. Aspartate transaminase are high at birth.

But the rise in Alanine Transaminase is less.But the rise in Alanine Transaminase is less.

The serum bilirubin rises after birth and peaks The serum bilirubin rises after birth and peaks after about the 3after about the 3rdrd to 5 to 5thth day of life. day of life.

Plasma urea decreases after birth because the Plasma urea decreases after birth because the infant synthesizes new proteins. The plasma infant synthesizes new proteins. The plasma amino acids are also low as a result of the same amino acids are also low as a result of the same reason. reason.

Urinary excretion of amino acids may be high Urinary excretion of amino acids may be high because of the immaturity of the tubular because of the immaturity of the tubular reabsorptive mechanisms. reabsorptive mechanisms.

The plasma urate is high at birth but the large The plasma urate is high at birth but the large clearance of urate soon reduces the plasma clearance of urate soon reduces the plasma concentration below the adult value. concentration below the adult value.

The serum Thyroxine of the healthy newborn is The serum Thyroxine of the healthy newborn is appreciably high. This physiological appreciably high. This physiological hyperthyroidism declines over the first year of hyperthyroidism declines over the first year of life. life.

The blood glucose in neonates is low because of The blood glucose in neonates is low because of small glycogen reserves. small glycogen reserves.

Blood lipids are low but reach 80% of adult Blood lipids are low but reach 80% of adult values two weeks post-partum. At 12 hrs post-values two weeks post-partum. At 12 hrs post-partum it decreases below the adult value partum it decreases below the adult value before rising again to a value slightly higher before rising again to a value slightly higher than in an adult. This change is largely due to than in an adult. This change is largely due to fluid transfer in and out from the capillaries. fluid transfer in and out from the capillaries.

Plasma potassium may be as high as 7 Plasma potassium may be as high as 7 mmol/l at birth (Normal= 3.5-5.0mmol/l) mmol/l at birth (Normal= 3.5-5.0mmol/l) but rapidly falls thereafter. but rapidly falls thereafter.

Plasma Calcium is also initially high but it Plasma Calcium is also initially high but it falls rapidly to 0.35mmol/l (Normal =2.1-falls rapidly to 0.35mmol/l (Normal =2.1-2.6mmol/l) during the first day of life.2.6mmol/l) during the first day of life.

Childhood- puberty: Childhood- puberty: The serum activity of The serum activity of most enzymes reduces during childhood to adult most enzymes reduces during childhood to adult values by puberty or earlier. values by puberty or earlier.

Although the activity of ALT at least in males Although the activity of ALT at least in males until the middle age, serum alkaline until the middle age, serum alkaline phosphatase correlates with skeletal growth and phosphatase correlates with skeletal growth and sexual maturity and its greater during sexual maturity and its greater during pregnancy, especially in females.pregnancy, especially in females.

The serum creatinine rises steadily from infancy The serum creatinine rises steadily from infancy to puberty prior to the development of skeletal to puberty prior to the development of skeletal muscle. The serum urate (uric acid) muscle. The serum urate (uric acid) concentration falls from the high level at birth concentration falls from the high level at birth until about 7-10 years of sage at which time it until about 7-10 years of sage at which time it begins to rise especially boys until at about 16 begins to rise especially boys until at about 16 years of age.years of age.

The young adult:The young adult: The concentration of The concentration of most constituents remains quite constant most constituents remains quite constant between puberty and the middle age in between puberty and the middle age in men and puberty to menopause in men and puberty to menopause in women. women.

The serum total cholesterol and The serum total cholesterol and triglycerides concentration increases in triglycerides concentration increases in both males and females at a rate of both males and females at a rate of 0.02mmol/l annum to a maximum 0.02mmol/l annum to a maximum between 50 to 60 years.between 50 to 60 years.

The elderly adult:The elderly adult: significant increase in the significant increase in the plasma concentration of blood constituents plasma concentration of blood constituents occur in women after menopause. occur in women after menopause.

Urea concentration rises with age. Urea concentration rises with age.

Hormone concentrations are affected by age Hormone concentrations are affected by age e.g. T3 concentration falls by up to 40% in e.g. T3 concentration falls by up to 40% in people over 40 years. people over 40 years.

Secretion and concentration of testosterone are Secretion and concentration of testosterone are decreased in males after 50 years. decreased in males after 50 years.

In women, serum concentration of oestrogen In women, serum concentration of oestrogen decreases by about 70% or more after decreases by about 70% or more after menopause. menopause.

The International System of Units (SI Units) has been The International System of Units (SI Units) has been developed and agreed internationally in the interests developed and agreed internationally in the interests of world health. It overcomes language barriers, of world health. It overcomes language barriers, enabling an exchange of health information within a enabling an exchange of health information within a country and between nations to be made without the country and between nations to be made without the misunderstandings which arise when each country, misunderstandings which arise when each country, or even a separate hospital within a country, uses its or even a separate hospital within a country, uses its own units of measurement for reporting tests.own units of measurement for reporting tests.

The SI units are based on the metre- kilogram-The SI units are based on the metre- kilogram-second system and replace both the foot-pound- second system and replace both the foot-pound- second (imperial) system and the centimeter-gram-second (imperial) system and the centimeter-gram-second (cgs) system. There are seven SI-based units, second (cgs) system. There are seven SI-based units, i.e. metre, kilogram, second, mole, ampere, Kelvin i.e. metre, kilogram, second, mole, ampere, Kelvin and candela.and candela.

UNITS USED IN REPORTING LABORATORY UNITS USED IN REPORTING LABORATORY RESULTSRESULTS

Symbols for these units and what they Symbols for these units and what they measure are:measure are:

SI-base units’SI-base units’ UnitsUnits Quantity Quantity MeasuredMeasured

MetreMetre MM LengthLength

KilogramKilogram KgKg MassMass

SecondSecond SS TimeTime

MoleMole MolMol Amount of Amount of substancesubstance

AmpereAmpere AA Electric currentElectric current

KelvinKelvin kk TemperatureTemperature

CandelaCandela CdCd Luminous Luminous intensityintensity