Table 2. Common blood tests, factors that affect the pre-analytical phase, and key recommendations.

Name of test	Factors affecting the pre-analytical phase	Recommendations
Aldosterone	Posture Medications [NB1] Timing of collection	Best time to collect is mid-morning. Indicate if sample was taken with patient upright or supine.
Blood film	Sample time in tube	Red cell morphology may change with prolonged time in tube. Prepare blood films on-site if delay is anticipated.
Blood gases	Temperature	Place on ice and transport to lab immediately.
Coagulation studies	Overfilled or underfilled sample tube	Fill the sample tube to the indicated line.
Cortisol	Diurnal variation Corticosteroid use Combined oral contraceptive pill Oral estrogen Pregnancy	Serum cortisol (for hypocortisolism) should be taken between 8 and 10 am. Corticosteroids suppress endogenous cortisol and should be stopped 24 to 72 hours before testing. Oral estrogen raises cortisol-binding globulin and thus cortisol concentrations, and should be stopped for 6 weeks before testing.
Creatinine	Trimethoprim Cephalosporins (some assays only) Creatine supplements	Stop offending agents if feasible and allow washout before retesting. Contact your laboratory for advice.
CrossLaps (C-terminal telopeptide)	Fasting status	Ensure patient has fasted before collection.
Fasting glucose	Fasting status Collection tube Delayed separation	Fast for at least 8 hours. Use appropriate collection tube (check with laboratory). Process samples promptly.
Iron	Diurnal variation Fasting status	Fasting samples are preferred, as results can be falsely elevated if not fasted.
Lipid studies	-	Routine fasting is not required.
Plasma metanephrines	Posture Caffeine Medications (MAOIs, SNRIs, SSRIs, TCAs)	Allow patient to lie supine for 30 minutes prior to collection. Allow washout of interfering medications before testing.
Potassium	Haemolysis Delayed separation Leucocytosis Thrombocytosis EDTA contamination	Take measures to reduce haemolysis. Process samples promptly. Ensure correct tube use (do not pour blood from one tube to another).
Renin	Cryoactivation Medications [NB1]	Store and transport samples at room temperature.
Testosterone	Diurnal variation	Collect sample between 8 and 10 am, or as soon after waking as possible.
Thyroid function tests	Biotin	Discontinue biotin-containing supplements or products at least 1 week before testing.
Urea	Dehydration	Ensure patient is adequately hydrated before sample collection.
Vitamin A	Photolability	Protect from light (wrap in aluminium foil).
Vitamin C	Photolability Temperature	Protect from light (wrap in aluminium foil). Place on ice immediately after collection.

EDTA = ethylenediaminetetraacetic acid; MAOIs = monoamine oxidase inhibitors; SNRIs = selective noradrenaline reuptake inhibitors; SSRIs = selective serotonin reuptake inhibitors; TCAs = tricyclic antidepressants

NB1: Medications that affect renin and aldosterone concentrations include mineralocorticoid receptor antagonists, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, beta blockers, dihydropyridine calcium channel blockers, thiazide and loop diuretics, methyldopa and clonidine. Antihypertensives that do not interfere include verapamil, prazosin, moxonidine and hydralazine.