Interference by Analgesic and Antirheumatic Drugs in 25 Common Laboratory Assays

Twenty five different analytical procedures, commonly used in clinical laboratories, were investigated for interference by eight analgesic and antirheumatic drugs. Ten of the investigated assays showed no statistically significant interference. Acetylsalicylic acid interfered in six assays (for glucose, uric acid, protein and cholesterol). Aminophenazone significantly decreased glucose, bilirubin and protein values, whereas caffeine affected four methods (for glucose, protein and iron). No definite influence of phenobarbital could be detected on any assay. Glucose, uric acid and iron values were altered in the presence of diclofenac. Indomethacin interfered in glucose, urea, uric acid and protein assays. Samples containing ibuprofen had altered creatinine, bilirubin and iron values, whereas ketoprofen interfered in glucose and iron determination.


Introduction
in the treatment of various rheumatic diseases. In our previous investigation, effects of analgesic and Analgetic preparations containing acetylsalicylic acid, antirheumatic drugs on the assay of serum enzymes aminophenazone, phenobarbital or cafieine are also (1) and SMA H procedures (2) were examined.
frequently used in self-medication. It is thus import-Acetylsalicylic acid, aminophenazone, indoinethacin, ant to know whether these drugs alter clinical labdiclofenac, ibuprofen and ketoprofen are widely used oratory test results.
In the present work, we investigated the effects of these eight drugs (in vitro) on 25 of the most frequently used methods for the determination of glucose, urea, uric acid, creatinine, bilirubin, protein, cholesterol, triglycerides and iron.
Tab. 1. Concentrations of added drugs in reconstituted lyophilized human sera.

Results and Discussion
The effects of analgesic and antirheumatic drugs on 25 common laboratory assays are shown in tables 3 -5. The concentrations of analytes and their variations obtained using different methods in samples with and without the drugs are given, s well s the corresponding p values. Ten assays showed no statistically significant interference, i.e. the determination of glucose (hexokinase method), urea (with glutamate dehydrogenase and phenol-hypochlorite method), uric acid (direct UV lest and phosphotungstate inethod), creatinine (continuous and Slofs method), bilirubin (Jendrassik-GroJ), cholesterol and triglycerides (enzymatic assays). No definite influence of phenobarbital could be detected on any method (tab. 4).
Acetylsalicylic acid interfered in six assays (for glucose, uric acid, protein and cholesterol determination) (tab. 3). Out of five glucose assays tested, two were affected by the drug: the glucose dehydrogenase method and the glucose oxidaseperoxidase procedure using ABTS s chromogen. Acetylsalicylic acid has been reported to have no in vitro effect on the alkaline ferricyanide, p-HBAH and o-toluidine methods (33). In sera containing the drug, apparent concentrations of uric acid (with uricase and catalase) and protein (direct spectrophotometric assay) were very significantly increased (p < 0.01 and p < 0.005, respectively). Wirth & Thompson (34) reported interference by acetylsalicylic acid in the Polin-Ciocalteu protein assay in body fluids. Lower cholesterol values were obtained using the Liebermann-Burchard method (p < 0.01), which is in agreement with findings reported by Caraway & Kammeyer (35).
Aminophenazone significantly decreased the results for glucose (with ABTS, p < 0.001), bilirubin (with dimethylsulphoxide, p < 0.05), and protein (biuret method, p < 0.05). The drug is also known to increase the results of the cholesterol determination based on reaction with ferric ions in acetic acid-sulphuric acid (33,35 Caffeine affected four tests (tab. 3). Glucose values were very significantly increased (ABTS method, p < 0.005), s well s protein (spectrophotometric assay, p < 0.001). Interference by caffeine in the uric acid assay using phosphotungstate has been reported (35), but the effect was found to be significant only in the determination of uric acid in urine.
Effects of diclofenac and indpmethacin are shown in table 4. In samples containing these drugs, the values for glucose (with ABTS) and uric acid values (with uricase and catalase and with aldehyde dehydrogenase) were elevated. Indomethacin affected very significantly the results of the glucose determination (with glucose dehydrogenase, p < 0.005), and the protein determination (biuret method, p < 0.005).
Creatinine (with deproteinization) and iron (Ramsay) values were increased in sera containing ibuprofen (tab. 5). Ketoprofen interfered in glucose (with ABTS, p < 0.02) and iron assays (with bathophenanthroline, p < 0.05 and Ramsay method, p < 0.005). No data have been previously reported on analytical interferences by diclofeiiac, indomethacin and ketoprofen.
' r The effects found were studied further using lower concentrations of drugs, down to the therapeutic levels. Interference by aminophenazone in tbe glucose determination (with ABTS) was still evident at 216 μιηοΐ of drug per liter (p < 0.025). Lower concentration of diclofenac (28 μπιοΐ/ΐ) significantly inĉ reased the results of the same test (p < 0.05). Figurel shows the d se dependence of these two interferences. In lower concentrations, other drugs did not interfere with any assay, so the effects found are significant only in cases of drug overdosage or poisoning.

Analyte, method
Glucose  Fig. 1. Dose dependence of aminophenazpne (a) and diclofenac (b) interference in the glucose assay with glucose oxidase and peroxidase using ABTS s chromogen.