Abstract
False elevations of plasma lactate dehydrogenase (LDH), potassium and aspartate aminotransferase (AST) have been described, in relation to haemolysis, occurring most often by mechanical release during phlebotomy or specimen processing. We present the cases of two leukaemic patients with severe hyperleukocytosis for whom LDH, potassium and AST were dramatically but falsely elevated. This false elevation was not caused by haemolysis but could be related to white cells lysis during transport through a pneumatic transportation system, enhanced by a specific fragility of leukaemic cells. Interestingly, this interference almost completely disappeared when serum rather than plasma was used, or when leukocytosis came back to normal. This work is meant to alert clinicians to the risks of errors in LDH, potassium and AST in leukaemic patients and suggest what precautions to take.
Introduction
Reports of pseudohyperkalaemia have been made in leukaemic patients, in the serum or even in the plasma. The causes have been variably attributed to diffusion of potassium from the buffy coat layer, heparin-mediated cell membrane damage, 1 mechanical disruption of white blood cells by pneumatic tube transport. 2 We describe here two cases of falsely elevated plasma potassium, lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) in leukaemic patients with major leukocytosis after pneumatic transportation. This interference disappeared when pedestrian transport was used instead of pneumatic transport, or when serum was used instead of plasma.
Case report
Case 1 was a 25-year-old patient admitted for chemotherapy after diagnosis of acute T-cell lymphoblastic leukaemia. White cell count (WBC) was 309 × 109 cells/L (90% blasts) and 86 × 109 platelets/L (reference range for WBC: 4–10 × 109 cells/L and for platelets: 150–400 × 109/L). The plasma potassium concentration obtained from a non-haemolyzed lithium-heparin tube transported to the laboratory via a pneumatic system was 9 mmol/L on Advia 2400 (Siemens Medical Solutions Diagnostics, Tarrytown, NY, USA). The patient presented no symptoms associated with hyperkalaemia. The possibility of pseudohyperkalaemia was investigated. Venous blood specimens were collected from a catheter both on lithium-heparin Vacutainer tubes and on red top Vacutainer tubes without anticoagulant. Tubes used were all 5 mL plastic tubes (diameter: 13 mm, length: 75 mm), they did not contain separator gel and were filled to capacity. Samples were sent to the laboratory about 15 min after phlebotomy, either via a pneumatic system or via pedestrian transport (<5 min both ways). In the pneumatic system used (Swisslog, Switzerland), tubes are inserted in a first disposable plastic bag which is placed in a second bag with parachutes propelled in a one-way air-flow system with speed limited to 6 m/s and soft landing capabilities. Upon receipt at the laboratory, tubes were promptly centrifuged (10 min at 2465
Measured K, AST and LDH concentrations, obtained in different specimens, using different transport routes
K (mmol/L), AST (UI/L), LDH (UI/L) concentrations and haemolysis index (semi-quantitative results) were determined on an Advia 2400 analyser; point of care analysis used i-STAT analysers; WBC was measured on a Sysmex XE-2100 analyser (Sysmex Corporation, Kobe, Japan); nr: no result Reference range for K: 3.5–5 mmol/L, AST: 15–40 UI/L, LDH: 220–480 UI/L and WBC: 4–10 × 109 cells/L, K, potassium; AST, aspartate aminotransferase; LDH, lactate dehydrogenase; WBC, white blood cell
Case 2 was a 15-year-old girl with acute T-cell lymphoblastic leukaemia (WBC: 358 × 109 cells/L, 95% blasts, 4 × 109 platelets/L), with no clinical signs of hyperkalaemia. Results were similar as in case 1 (data not shown) with high erroneous potassium, LDH and AST plasma values for specimens transported via the pneumatic system, and normal values when pedestrian transport point of care analysis, or serum was used. No sign of haemolysis was present on either specimen.
Additionally, haemolysis index, potassium, LDH and AST were evaluated in 59 non-leukaemic patients (WBC: 1.24–33.23 × 109 cells/L), after sampling in pairs on 5-mL lithium-heparin tubes and transportation either through the pneumatic system or by human porter. Measurements performed on both Advia 2400 and Modular DP did not show any of the abnormalities reported in the two described cases. In particular, potassium was not affected by the different transport processes, and nor was AST. However, when sent through the pneumatic system, mild haemolysis was more frequent (12% of cases versus 3.5% when delivered by porter), and so was mild elevation of LDH (data not shown), which will be investigated further.
Discussion
Pseudohyperkalaemia, in a reverse pattern (absent in serum), has been recently described in a leukaemic patient on a site using pneumatic transport, 1 but imputability to the pneumatic transport was not examined. Our results are in favour of the responsibility of the pneumatic transport, acting via mechanical disruption of heparin-weakened white cells, 1 from leukaemic patients with extreme leukocytosis, especially as leukaemic cells may inherently show increased fragility to lysis and leaky membranes. 3 This phenomenon may also cause errors in other analytes measurement such as LDH and AST, which are largely released with cell lysis. Interestingly, such interference is not observed when serum is used. We hypothesize that clotting process may protect the cells therein imprisoned from mechanical damage notably during pneumatic transport. Pseudohyperkalaemia is observed in association with clotted specimen, but gives comparatively minor rises. 4 Other hypothesis have been evoked such as heparin-mediated cell membrane damage. 1 Although no clear evidence, as far as we are aware, has been brought that heparin contained in tubes could have damaging effects on normal blood cells, we cannot rule out that it could exert some damaging effects on fragilized membranes. Centrifugation conditions and delays have been suggested as causes of false elevations of potassium. 5 In our case, centrifugation, performed according to the manufacturer's recommendation, does not appear to be a major cause of hyperkalaemia as delays never exceeded 20 min prior to centrifugation, and, following transport, both pedestrian and pneumatic transported tubes underwent the same process of centrifugation, showing normal potassium values in one case and false elevated values in the other. Premature centrifugation could also be cause of serum haemolysis by disruption of the clot, which was not observed here. Type of tubes (with or without separator gel) used to collect samples subjected to pneumatic transport, and tube size to a lesser extent, may play a role in susceptibility to haemolysis. 6 Reduced leakage of plasma potassium 1 in the presence of gel separator and a protective effect of separator gel on haemolysis in serum-gel tubes 6 were reported after pneumatic transportation. However, unexpectedly high and erroneous plasma potassium values have also been attributed to the presence of gel in the tube in another case of leukaemic patient. 7 Moreover, when sent through a pneumatic system, plain serum samples are more prone to haemolysis 6 than heparin ones. Curiously, false elevation of potassium was not observed in our case when plain serum samples were used, which matches what had been observed by Abraham et al. 1 Moreover, as we did not use serum-gel tubes, we cannot evoke a gel protecting effect in our case. Although the pneumatic system we use (Swisslog, Switzerland) is modern, properly maintained and used according to the manufacturer's recommendations, its weakness could well lie in the weak protection of the tubes during the run. 8 Tubes are not protected in solid carriers with foam padding or bubble wraps, they are just inserted in disposable plastic bags with parachutes propelled by compressed air. Moreover, the system configuration including multiple turns may cause sufficient trauma to lyse blood cells. Given the increased incidence of haemolysis in control subjects with normal blood count that pass through this system, the setting of this pneumatic transportation system will require further investigations. In those leukaemic patients with high white cell count, more reliable results require point of care analysis, pedestrian transport or analysis on serum. Possible effect of the use or not of separator gel should also be considered. Awareness of this phenomenon could avoid inappropriate diagnostic or therapeutic measures.
DECLARATIONS