Effects of drugs on chemical tests
Michaela Králíková, MD
Department of Biochemistry
Faculty of M e ill cine
Masaryk'University
Drugs may have specific effects on the plasma/serum or urine levels of many
substances.
• These effects are more apparent when large doses of a drug are administered for a long time than when administration of a single dose occurs on an isolated occasion.
► Drugs can either interfere with the laboratory tests in vitro or in vivo.
In vitro interferences
•  color during spectrophotometric measurement
•  chemical reaction with the analyte of interest or chemical substances used during analysis
•  complexes formation
•  Example: ascorbate or salicylate (reducing
agents) and glucose in urine
In vivo interferences
•  arise from the therapeutic intent of d rugs, their side effects, and patient condition and drug-response:
•   interference with production, metabolism or
elimination of the analyte (enzyme activation, inhibition or competition, transport mechanisms competition etc.)
•   drug injuries to a certain organ /tissue —> its function impairment
Interactions drug - diet
•  physiochemical — reciprocal interference with absorption or solubility, charge change, chelate and other complexes formation
•   Example: tetracyklin - Ca2+
•  physiological - i or t of appetite, influence on peristalsis
•  patophysiological - initiation of toxic effects of the drug by food
^^> Follow up of drug intake:
peroral antidiabetics
contraceptives
antihypertensive agents
salicylate
peroral anticoagulants
antibiotics
vitamines
diuretics etc.
Examples of drug interferences
•   ascorbate* salicylate streptomycin
t glc/U (non-specific reaction for reducing agents)
•   ascorbate
false positive gFOBT
•   theophyllin
determination of uric acid               i                 i
in viíľD interľšľšuzbs
Examples of drug interferences • corticosteroids:
•   f synthesis of glycogen (liver)
•   I proteosynthesis —► I enzyme activities
•   j glucose tolerance —► f glc/S
•   negative nitrogen balance (<— f protein catabolism)
•   t cleavage of TG —> f FA
•   j leucotriens synthesis (j arachidonic acid <— J, activity of phospholipase)
•   f resorption of Na+, CI" in renal tubules —► Na+ and (Xretention —► water retention
•   f renal secretion of K+, H+ —► | K+/S> alkalosis
in Yi 7 d in t srišľ m £ í&
Examples of drug inteif fences
•  morfium
non-specific | ALT, LD, AMS
•  contraceptives
t TG, Chol
•  furosemid
t glc, AMS, ALP, i Na
Therapeutic drug monitoring (TDM)
the measurement of drugs in body fluids as an aid to
controlling dosage
•   Is a benefit in assessing the therapeutic response or possible toxic side-effects.
•   Is a benefit where there is a clear difference between the therapeutic and toxic effects and where serum levels correlate well with therapeutic and toxic effects.
•   Examples: digoxin, theophylline, anticonvulsants
(Phenytoin, phenobarbitone, carbamazepine), lithium, aminoglykoside antibiotics