A simple method for excluding possible Citra-lock TM contamination

In response to the letter by Lamb and Abbas (Ann Clin Biochem 2007;44:579), ¹ we have observed that the chloride concentrations are within the reference range for samples contaminated with Citra-lock™. This is similar to the results seen with samples contaminated with sodium citrate from vacutainers. Consequently, a raised chloride concentration in conjunction with a raised sodium could be used to exclude Citra-lock™ contamination.

The measured osmolality of samples contaminated with sodium citrate from vacutainers is also inappropriately low compared with the calculated osmolality, whereas the measured osmolality in samples contaminated with Citra-lock™ is raised. This can serve to differentiate the two situations (Table 1).

Table 1

Effect of sodium citrate contamination on laboratory results

Analyte (units)	Blood sample contaminated with buffered sodium citrate from a vacutainer	Blood sample contaminated with Citra-lockTM
Sodium (mmol/L)	161	162
Potassium (mmol/L)	3.9	4.2
Chloride (mmol/L)	76	98
Bicarbonate (mmol/L)	16	23
Urea (mmol/L)	9.6	13.1
Osmolality (mOsmol/kg)	305	330
Calculated osmolality 2([Na] + [K]) + [urea]	339	346

The lower than expected osmolality for samples containing sodium citrate is due to the fact that for every 3 mmol of sodium there is only 1 mmol of citrate. The higher osmolality with Citra-lock™ contaminated samples is likely to be related to the high concentration of sodium citrate present in Citra-lock™ such that there is very little dilution of the original sample. According to the article by Srivastava and Murphy et al. ² only 10 μL of Citra-lock™ is sufficient to increase the serum sodium concentration by approximately 20 mmol/L.