This site is intended for health professionals only

At the heart of general practice since 1960

Measuring plasma, serum, or whole blood concentrations of drugs

Drug Blood Ideal time Usual target Usual target Factors that alter Factors that alter response

sample range (mass units) range (molar units) blood concentration at a constant

at a constant dosage blood concentration

Ciclosporin Whole blood Just before 100-250ng/ml 80-200nmol/l Diarrhoea, reduced bile

next dose salt production, hepatic

insufficiency, drug

interactions (eg, ketoconazole,

St John's wort)

Digoxin Plasma or 11 hours after 0.8-1.5ng/ml 1.0-2.0nmol/l Renal insufficiency; drug Hypokalaemia, thyroid

serum last dose interactions (eg, amiodarone, disease, drug

verapamil), thyroid disease interactions (eg, diuretics)

Lithium Serum 12 hours after - 0.4-0.8mmol/l Renal insufficiency, diarrhoea,

last dose changed formulation, drug

interactions (eg, diuretics)

Phenytoin Plasma or Just before 10-20µg/ml 40-80µmol/l Drug interactions (eg, Altered protein binding,

serum next dose vigabatrin, valproate) other antiepileptic drugs

(additive effects)

Theophylline Plasma or Just before 10-20µg/ml 55-110µmol/l Smoking, hepatic insufficiency,

serum next dose drug interactions

(eg, erythromycin, quinolones)

Rate this article 

Click to rate

  • 1 star out of 5
  • 2 stars out of 5
  • 3 stars out of 5
  • 4 stars out of 5
  • 5 stars out of 5

0 out of 5 stars

Have your say