Our study showed that high CrCl is often seen on admission in ICU patients with community-acquired acute infectious meningitis. It can persist for several days and be predicted by the estimated methods.
CrCl is classically screened for GFR despite limitations. Creatinine is metabolised and secreted by the renal tubule, and hence CrCl both overestimates the GFR and delays the diagnosis of acute kidney injury. Inulin clearance is the gold standard test for accurately measuring GFR. However it is complex to perform, highly expensive, and cannot therefore be routinely used in ICU patients. Iohexol clearance is an alternative to estimating GFR. However, the use of iodine in the test probably precludes its sequential and repeated use in patients and in healthy volunteers. In clinical practice, despite its limitations, CrCl is currently used to estimate GFR because it is inexpensive, harmless, and easy to perform, particularly in ICU patients. Moreover, it is now well established that small plasma creatinine variations are independently associated with poor prognosis in ICU patients
, reinforcing the widespread measurement of plasma creatinine for the diagnosis and follow up of kidney function in these patients.
There is no consensus on what constitute normal GFR values, but most authors consider them to range between 90 and 130 ml/min/1.73 m2. Similarly, the normal values of CrCl are not agreed upon and most authors extrapolate them from those defined for GFR. Several threshold values have been used to define high CrCl, ranging from 120 ml/min/1.73 m2 to 150 ml/min/1.73 m2 for women and 160 ml/min/1.73 m2 for men
[10–13]. In light of all these factors, we chose a 140 ml/min/1.73 m2 threshold to define high CrCl. This probably allows a greater specificity in the diagnosis of high CrCl than in most studies. We made no distinction between male and female patients since gender entails no difference in treatment, in particular with regard to dosage. Cockcroft-Gault formula and simplified MDRD equation are commonly used to determine CrCl because diuresis quantification and urinary creatinine measurement are not necessary in the equations. Hence, estimation of CrCl by these two methods is simpler than calculation by 24-hr UV/P creatinine. CrCl as estimated from equations has been recommended for dosage adjustment in patients with reduced kidney function receiving antibiotics with renal elimination. However, no consensus firmly states which methods must be used in ICU patients to determine CrCl. In our study we defined high CrCl as clearance above 140 ml/min/1.73 m2 by 24-hr UV/P creatinine. As reported elsewhere, our results demonstrate that CrCl calculated with the 24-hr UV/P creatinine and estimated from equations may yield different results
[3, 12, 14]. Baptista et al. reported that in patients with augmented CrCl, the estimated methods of CrCl significantly under-estimate the measured value of the calculated method
. Our findings are in agreement with previously published data and suggest that the estimated methods of CrCl could be used as a screening tool to diagnose high CrCl with a 101 ml/min/1.73 m2 threshold for Cockcroft-Gault formula and a 108 ml/min/1.73 m2 threshold for simplified MDRD equation. The simplified MDRD equation and the Cockcroft-Gault formula make it possible to rule out a high CrCl when their values are under their respective thresholds.
There is a paucity of information in the literature relating to the characteristics of high CrCl in ICU patients. The incidence of high CrCl in a general population of ICU patients can vary between 18%
 and 41%
. The main factors associated with high CrCl in these studies are young age, greater body surface area, higher diastolic blood pressure, diuresis levels, lower APACHE III scores at admission and multiple trauma
[10, 11]. In a small sample of patients with head trauma, the incidence of high CrCl was 85%
. In addition, some studies that did not specifically deal with high CrCl but either with the assessment of renal function in particular ICU patient groups (burns, trauma, brain injury, surgery)
[2–4, 10], or with the pharmacokinetic and pharmacodynamic properties of antibiotics
[15–18], recorded an incidence of high CrCl ranging from 3% to 50% and values between 130 and 200 ml/min/1,73 m2.
Little is known about the pathological mechanisms of high CrCl in ICU patients. The condition has been reported during pregnancy, following nephrectomy, at the onset of diabetic kidney disease, in obesity, in sickle cell anaemia
 and in children with haemopathy
. It has been suggested that high CrCl occurs as a result of glomerular hyperperfusion
. Experimental studies have shown that high CrCl is observed during the initial hyperkinetic phase of sepsis
[22, 23]. However, this hypothesis cannot account for all cases of high CrCl in acute or chronic situations. Another suggested explanation for high CrCl when it is detected at the onset of diabetic kidney disease is that hyperglycaemia stimulates the re-absorption of sodium in the proximal tubule thereby decreasing its supply to the distal tubule. This would set off tubuloglomerular feedback in the macula densa and result in dilatation of the afferent arteriole and an increase in GFR
. A third explanation, in particular in children with malignancies and in obese patients is that high CrCl is due to the kidney adjusting to the high protein load caused by large tumours
 or to the hypermetabolism brought on by obesity
. No single hypothesis of the above three seems to explain the high CrCl observed in our study, and it is likely that several mechanisms occur in combination.
In drugs with kidney clearance, a decrease or increase in normal GFR can have a major impact on its plasma concentration. Lowered GFR impairs elimination of the drug and can lead to excessively high concentrations if the dosage is not decreased or if the intervals between doses are not lengthened. Drug overdosage can have serious side effects resulting in increased morbidity
[15, 26] and for this reason there are dosage guidelines to be followed in the event of a decrease in GFR. Conversely, high GFR can lead to greater concentrations of the drug being eliminated with the possible risk of insufficient dosage and hence treatment failure or, in the case of antibiotics, the emergence of resistant bacteria. There are currently no guidelines for adjusting drug dosage when GFR is high. Our study is not able to assess the pharmacokinetic or pharmacodynamic effects of a drug during high CrCl. Assessments of these effects elsewhere have shown that there is a close connection between the plasma concentration of a drug and CrCl and that, for example, high CrCl is associated with low plasma concentrations of the drug
[15, 16, 27–29]. This suggests that high CrCl could lower drug concentration to a level that renders it therapeutically ineffective, despite a seemingly appropriate dosage. No study has shown a causal relation between high CrCl and failure of drug treatment. Our study shows that high CrCl can persist for several days. It is possible that when high CrCl lasts only a few hours it has little impact clinically whereas a longer period of high CrCl would probably require daily dosage adjustment. It would be interesting to perform a daily monitoring of the presence and intensity of high CrCl in patients taking drugs excreted by the kidneys.
Our study has several limitations. First, it was retrospective and therefore we cannot rule out the possibility that certain eligible patients were excluded because of an oversight in the computer rating of diagnosis of meningitis. However, the main outcome criterion, the incidence of high CrCl, did not influence the selection of the patients recruited. Second, it was a monocentric study including only ICU patients admitted for meningitis, and so extrapolating the results to a general ICU population remains speculative. The study was focused on patients with meningitis, since in this clinical situation, prompt antiinfectious treatment with both adequate agent and dosage is mandatory for preventing poor outcome. High CrCl may impact the pharmacokinetics of the drugs resulting in therapeutic failure due to low drug concentration despite an apparent appropriate drug dosage. Third, the diagnosis of meningitis was based on CSF white cell count, and therefore may have been overestimated. However, this diagnostic criterion is classically used in studies on meningitis in ICU patients and the magnitude of the results would not have been different if we had limited the study to patients with meningitis defined by a CSF culture yielding a microorganism. Fourth, CrCl rate can be affected by certain drugs, such as cimetidine
 and trimethoprim
, that stimulate tubular secretion of creatinine. No such drugs were administered in our cohort.