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Plasma osmolality reference values in African grey parrots (psittacus erithacus erithacus), Hispaniolan Amazon parrots (Amazona ventralis), and red-fronted macaws (Ara rubrogenys).

Abstract: Birds are routinely presented to veterinarians for dehydration. Success with these cases ultimately depends on providing replacement fluids and re-establishing fluid homeostasis. Few studies have been done to determine reference ranges for plasma osmolality in birds. The goals of this study were to determine reference values for plasma osmolality in 3 species of parrots and to provide recommendations on fluid selection for replacement therapy in these species. Blood samples were collected from 21 adult Hispaniolan Amazon parrots (Amazona ventralis), 21 Congo African grey parrots (Psittacus erithacus erithacus), and 9 red-fronted macaws (Ara rubrogenys), and were placed into lithium heparin containers. Plasma osmolality was measured in duplicate with a freezing point depression osmometer. Summary statistics were computed from the average values. Reference ranges, calculated by using the robust method, were 288-324, 308-345, and 223 369 mOsm/kg in African grey parrots, Hispaniolan Amazon parrots, and red-fronted macaws, respectively. The mean [+ or -] SD values were 306 [+ or -] 7, 327 [+ or -] 7, and 304 [+ or -] 18 mOsm/kg in African grey parrots, Hispaniolan Amazon parrots, and red-fronted macaws, respectively. Comparisons with osmolality values in mammals and values previously reported for psittacine bird species suggest that plasma osmolality is slightly higher in parrots than in mammals, species-specific differences exist, and differences between reported values occur. Overall, fluids with an osmolarity close to 300-320 mOsm/L, such as Normosol-R, Plasmalyte-R, Plasmalyte-A, and NaCl 0.9%, can be recommended in parrots for fluid replacement therapy when isotonic fluids are required.

Key words. plasma osmolality, osmolarity, fluid therapy, psittacine, avian, African grey parrot, Psittacus erithacus erithacus, Hispaniolan Amazon parrot, Amazona ventralis, red-fronted macaw, Ara rubrogenys


The dynamics of body fluids between compartments and across membranes depend on many factors and are based on the composition and osmotic pressure of each compartment. Osmolality is defined as the concentration of solutes in moles per kilogram of solvent (mOsm/kg). Osmolarity, however, is the concentration of solutes per liter of solution (mOsm/L). (1,2) Osmolality and osmolarity are considered equal in biologic fluids and can be used interchangeably, (2,3)

Plasma or serum osmolality can be used to infer osmotic and electrolyte disorders that an animal experiences in response to a disease process and a loss of water and electrolytes. The classification of fluids into isosmotic, hyperosmotic, and hypoosmotic is based on whether the solutions have a higher or lower osmolality than body fluids. (3) The classification into isotonic, hypertonic, and hypotonic is based on whether the solutions will cause a change in cell volume. (3) Tonicity and osmolarity are related but not interchangeable. For crystalloids, tonicity and osmolarity are roughly the same. Commercial fluid solutions have been determined based on human osmolality and osmoregulation and, although appropriate in domestic mammals, may not be appropriate in birds. A thorough knowledge of normal osmolality in psittacine bird species, therefore, is needed to formulate a fluid therapy plan in sick individuals where the goals are to restore physiologic osmolality and solutes concentration and to minimize iatrogenic effects.

Serum or plasma osmolality can be measured by osmometry. It also can be estimated by using formulas. The difference between the measured and calculated osmolality is called the osmolar gap and can be clinically useful to assess certain toxicities. (1,2) In clinical osmometry, the freezing point depression method is more routinely used, because it is more precise and more accurate than the vapor pressure depression method. The freezing point depression method is based on the property that the freezing point of a solution will be 1.86[degrees]C (35.3[degrees]F) lower than pure water for each mole/kg of a solute added. (1)

The goal of this study was to determine reference values for plasma osmolality in 3 species of parrots, each representing a group of psittacine birds commonly kept in captivity. This information is important to determine the most suitable fluid to use in birds in a fluid therapy plan. The specific biologic hypotheses in this study were that parrots have a higher plasma osmolality than domestic mammals and humans, and that fluids with a higher osmolarity are needed for replacement therapy in psittacine bird species.

Materials and Methods


Twenty-one adult Hispaniolan Amazon parrots (Amazona ventralis), 21 Congo African grey parrots (Psittacus erithacus erithacus), and 9 red-fronted macaws (Ara rubrogenys) were used in this study. The Amazon parrots were from an established colony housed at Louisiana State University, the African grey parrots were from a private breeder in Gautier, MS, USA, and the macaws were from the BREC (Recreation and Park Commission for the Parish of East Baton Rouge) Zoo (Baton Rouge, LA, USA). The Amazon parrots were housed indoors and fed a pelleted diet (Kaytee Exact Rainbow, Kaytee, Chilton, WI, USA), and the African grey parrots, and the macaws were housed outdoors and fed a pellet-seed mixture. The parrots were considered healthy based on an external physical examination and, for the Amazon parrots, results of a recent complete blood cell count and plasma biochemical profile. The study was approved by the Louisiana State University Institutional Animal Care and Use Committee.

Sample collection and osmometry

Each parrot was manually restrained. A 0.5-mL blood sample was collected from the right jugular vein by using a 26-gauge needle on a 3-mL syringe. Immediately after collection, blood samples were placed into a lithium heparin tube (Microtainer, Becton Dickinson and Co, Franklin Lakes, N J, USA) and centrifuged at 500g for 15 minutes. Plasma was harvested and placed in a cryogenic vial (Cryovial, Simport, Beloeil, QC, Canada) on ice. Vials were subsequently frozen at -30[degrees]C (-22[degrees]F) until processed. Within the next 5 days, plasma samples were thawed, and plasma osmolality was measured in duplicate from the same cryogenic vial by use of a commercial freezing-point depression osmometer (Osmette, Precision Systems, Natick, MA, USA). The osmometer was calibrated by use of a 2-point calibration technique in accordance with the manufacturer's instructions. The 2 osmolality measurements from each parrot were used to calculate a single average value.

Statistical analysis

Average values for the duplicate osmolality measurements were tested for normality by using skewness, kurtosis, quantiles plots, and the Shapiro-Wilk tests. Homogeneity of variances was assessed by the Levene test. Mean, SD, and coefficient of variation were determined for each species. Species means were compared by analysis of variance by using the mixed linear models procedure. Post hoc comparisons were obtained by the least square of means method with a Tukey adjustment. Species means also were compared with previously published means by Lumeij and Overduin (4) by using a 1-sample t test. In addition, the mean plasma osmolality of red-fronted macaws was compared by a 1-sample t test, with means published by Polo et al (5) for different Ara species. Finally, the overall mean for the birds was compared by a 1-sample t test with published means for different mammalian species. (3,6-8) An level of .05 was used for statistical significance. A Bonferroni adjustment was used when the osmolality mean value for one species was compared with several means from previously published articles ([alpha] = .012 for macaw comparisons, [alpha] = .017 for the overall mean). For nonsignificant differences, the post hoc power of the test was computed and was considered low when inferior to .8.

Reference values were determined by the robust method as described in the guidelines set by the Clinical and Laboratory Standards Institute. (9) For smaller sample sizes (fewer than 120), the robust method with calculation of a 95% reference range with a 90% confidence interval of the reference limits should be considered and shows good performance. (9-11) The reference ranges were determined as the interval between the 90% lower confidence limit of the lower 95% reference limit and the 90% upper confidence limit of the upper 95% reference limit. SAS (SAS 9.1.3, SAS Institute Inc, Cary, NC, USA) was used for statistical analysis. MedCalc (Version 11.3, MedCalc Software, Mariakerke, Belgium) was used for the reference range determinations.


The mean values of the duplicate osmolality measurements were normally distributed (P = .23), and variances were homogeneous across species (P = .07). Statistical analysis and reference values are presented in Table 1. The overall mean [+ or -] SD for plasma osmolality for the 3 psittacine bird species was 314 [+ or -] 14 mOsm/kg. The mean value was significantly different among the 3 species (F = 29.2, P < .001). Post hoc multiple comparison procedures revealed no significant difference between mean plasma osmolality of African grey parrots and red-fronted macaws (P = .88); however, the mean plasma osmolality of Hispaniolan Amazon parrots was significantly higher than that of the 2 other species (P < .001) with a mean difference of 20.8 and 22.7 mOsm/kg with the African grey parrots and red-fronted macaws, respectively. The power for the analysis of variance test was 0.995.


In this study, reference ranges were determined for plasma osmolality in 3 species of parrots (Table 1) and suggest that plasma osmolality is slightly higher in parrots than in mammals, with the presence of species-specific differences. The methodology used in this study included the use of heparinized containers and freezing of the plasma. Serum, rather than plasma, is usually preferred for osmometry because the addition of anticoagulants in plasma samples may increase solutes in the samples. (1-2) However, plasma is most commonly used in practice, and the difference between serum and heparinized plasma osmolality has been shown to be nonsignificant in humans. (8) Storage conditions also can alter the plasma osmolality. However, freezing does not induce a significant alteration in osmolality within 14 days of collection in human plasma samples. (8)

The results of this study suggest species-specific differences, with the plasma osmolality of Hispaniolan Amazon parrots higher than that of the 2 other species. However, there were some major differences in housing (indoors versus outdoors), diet (pellets versus seeds), and water availability (water bottle [Lix-it system, Lixit Corporation, Napa, CA, USA] versus bowl) between the Hispaniolan Amazon parrots and the other 2 species that may have accounted for the differences observed. Also, the reference range for the red-fronted macaws is wide and might reflect the smaller sample size used for this species.

Additional comparisons were made with previously published means for these species and selected mammals. The mean values of plasma osmolality in African grey parrots, Hispaniolan Amazon parrots, and red-fronted macaws were significantly lower than previously published means of 332 mOsm/kg (t = -17.29, P < .001), 334 mOsm/kg (t = -4.63, P < .001), and 341 mOsm/kg (t = -6,15, P < .001), respectively. (4) Lumeij and Overduin (4) reported no significant differences among osmolality of psittacine bird genera. The sample sizes were much higher for African grey parrots (70) and Amazon parrots (96) but similar for macaws (11). (4) Despite a higher sample size, the variability of measurement was greater in the study by Lumeij and Overduin (4) than in ours, with a SD of 8.8 mOsm/kg in African grey parrots, 15.4 mOsm/kg in Amazon parrots, and 18 mOsm/kg in macaws. The larger SD for the macaws and Amazon parrots may reflect the fact that the values are genus specific and not species specific. The osmometry technique also was not disclosed. Although some differences were statistically significant, they were mild and likely of no clinical significance.

Furthermore, the mean plasma osmolality in red-fronted macaws was significantly lower than previously published values by Polo et al (5) for scarlet macaws (Ara macao) (331 mOsm/kg; t = -4.49, P = .002) but similar to the published values for blue and gold macaws (Ara ararauna) (319 mOsm/kg; t = -2.50, P = .034), the green-winged macaw (Ara chloroptera) (315 mOsm/kg; t = -1.84, P = .10), and the military macaw (Ara militaris) (303 mOsm/kg; t = 0.15, P = .89). (5) The powers of comparison t tests for the blue and gold, green-winged, and military macaws were low, at 0.38, 0.20, and 0.02, respectively. No inference could be made on the lack of significant differences with blue and gold, green-winged, and military macaws because of the low power of the statistical tests. The sample size was similar (range, 8-17) as well as the variability (SD range, 6.2-30.8 mOsm/kg). (5)

The overall mean [+ or -] SD of 314 [+ or -] 14 mOsm/kg for the 3 psittacine birds species was significantly higher than the mean in dogs (300 mOsm/kg; t = 6.97, P < .001) and humans (282 mOsm/kg; t = 15.92, P < .001) but not significantly different from the mean in cats (310 mOsm/kg; t = 2.00, P = .05). The power computed for the cat comparison was low, at .49, and a significant difference may be found if a higher number of birds is used for this comparison.

The differences observed across studies may reflect a difference in sample size, laboratories, osmometry techniques, sample processing, and captive conditions of the birds. When considering the variability reported among studies, caution should be taken in interpreting these results. For cockatoos, the reported mean osmolality was 330 mOsm/kg in Cacatua species and 336 mOsm/ kg in the galah (Cacatua roseicapella). (4,12)

Overall, the results of this and previous studies show a trend toward higher osmolality in psittacine bird species compared with mammals, especially with humans in which the plasma osmolality is quite low (282 mOsm/kg) and on which most prepackaged fluids are based. The mild departure (5%-10%, depending on comparison group) observed in psittacine bird plasma osmolality from mammals is of relatively low magnitude and may not be of great clinical relevance for fluid replacement therapy. Nevertheless, selecting fluids with a slightly higher osmolarity may be warranted to minimize iatrogenic adverse effects. A comparison of different fluids (13,14) finds that Normosol-R (Abbott Laboratories, North Chicago, IL, USA), Plasmalyte-R (Baxter Healthcare Corp, Deerfield, IL, USA), Plasmalyte-A (Baxter Healthcare), and NaCl 0.9% have osmolarities that range from 300 to 310 mOsm/L and might be best for use in fluid replacement therapy to restore a more appropriate physiologic osmolality in dehydrated birds because they are approximately isosmolar to parrots. Alternatively, elaboration of a balanced replacement fluid with a higher osmolarity, closer to 310-315 mOsm/L, may be useful in birds and can easily be achieved by mixing different crystalloid fluids (eg, 500 mL of a 310 mOsm/kg fluid can be obtained by mixing 490 mE of Normosol-R and 10 mL of 3% NaCl).

In avian and mammalian body fluids, solutes that have the major contribution to osmolality are sodium, chloride, potassium, bicarbonates, glucose, urea, and uric acid. (1,15) Osmoregulation is the regulation of the balance of water and electrolytes, and is of primary importance in the animal's overall homeostasis to maintain a constant fluid volume and composition in each compartment, which is achieved through a balance between intake of water, solutes, and nutrients, and excretion of these substances and their end-products. (2) In birds, osmoregulation is tightly controlled by the kidneys, intestinal tract (coprodeum, caecum, and colon), salt glands, skin, and respiratory system. (15,16) Among these, the kidneys and salt glands (when present) are the primary regulatory organs. (15) Avian osmoregulation shares similarities with its mammalian counterpart but also presents some major differences. (15) Unlike mammals, birds may increase their plasma osmolality in response to water deprivation and may tolerate higher plasma osmolality. (17) Also, birds have the ability to reduce their glomerular filtration rate (GFR) and increase the tubular reabsorption of water in response to dehydration. (15) Conversely, birds are able to increase their GFR in response to extracellular fluid expansion induced by fluid therapy. (15) Because the elimination of uric acid occurs by tubular secretion, it is not compromised by a reduced GFR, thus, the constraints to maintain a constant GFR is less in birds than in mammals. (17) Also, various species respond differently to dehydration and water deprivation based on their geographical habitat of origin, consequently they may respond differently to fluid replacement therapy. For example, xerophilic psittacine bird species such as the galah and the budgerigar (Melopsittacus undulatus) possess osmoregulatory mechanisms that allow them to conserve water and to increase their plasma osmolality. (18,19)

Finally, fluid therapy should be adjusted to the needs and nature of the disease process of avian patients. Maintenance fluid therapy is given to hospitalized animals that have a correct hydration but are not able to sustain normal fluid balance by drinking. (13) The daily fluid maintenance in parrots is estimated from 50 mL/kg per day (15,20,21) to 100 mL/kg per day (S. Orosz, unpublished data, 2010). Maintenance fluids (eg, Plasma-Lyte 56, Baxter) are typically of lower osmolality, sodium, and of higher potassium content than the plasma and are given at low volumes. (13) Because of the low tolerance of intravenous or intraosseous catheters by active parrot patients, maintenance fluid therapy is most often administered by the subcutaneous route. (21) Most maintenance fluids cannot be given subcutaneously because of their hypotonicity; therefore, balanced replacement fluid solutions that are isotonic are used instead at a dose that does not exceed 10 mL/kg per site. (13) However, when possible in pet birds, replacement fluid therapy is frequently given via intravenous or intraosseous, because these routes provide rapid dissemination of water and electrolytes at a more precise dose. The subcutaneous route is not appropriate for severely dehydrated birds or patients in shock, because peripheral vasoconstriction will reduce their ability to absorb fluids given in this manner. (13) The intravenous or intraosseous routes also are used during anesthesia and surgery to maintain blood pressure and tissue perfusion, correct expected fluid loss during surgery, and maintain a vascular access for emergencies. Balanced replacement fluids that are isotonic to birds should be used during a surgical procedure. These solutions may be used in large volumes to expand the extracellular compartments without inducing changes in its composition and osmolality. Only 20% to 25% of the administered volume remains within the mammalian intravascular space after 1 hour. (22) In mammals and humans, large infusions of lactated Ringer's solution (LRS), but not NaC1 0.9%, induce a mild decrease in plasma osmolality and an increase in cerebral water. (23,24) The changes induced by LRS have the potential to be greater in animals with higher plasma osmolality (eg, parrots). Although avian fluid therapy is discussed here primarily because it relates to fluid osmolarity, fluid composition should also be considered.

The values reported in this study for Hispaniolan Amazon parrots also were used in a recent validation study. (25) This study demonstrated that calculated osmolality when using the equation 2 x ([Na.sup.+] + [K.sup.+]) + uric acid/16.8 + glucose/18 (SI units) had good agreement with the measured osmolality. Although the fit of this equation was not evaluated across various electrolytes and osmolality disorders, it appears that it provides a reasonable estimation of the osmolality in Amazon parrots and probably other parrot species. Therefore, it also can be applied to calculate the osmolar gap. (25) In psittacine bird species, the osmolar gap has the same theoretical use as in mammals for diagnosing certain toxicoses, but the occurrence of these types of toxicoses in pet birds (eg, ethylene glycol, methanol, paraldehyde) is rare and largely unreported, except in anseriformes and galliformes. (26-28)

The results of this study suggest that plasma osmolality in psittacine birds is slightly higher than that of domestic mammals, that species-specific differences exist, and that variations across reported values are present. Overall, the selection of fluids with an osmolarity of 300-320 mOsm/L can be recommended in parrots for fluid replacement therapy to avoid potential adverse effects.

Acknowledgments: We thank the South Alabama Bird Club and the Gulf South Bird Club for supporting this study. We also thank the Kaytee Avian Foundation for supporting the LSU Hispaniolan Amazon parrots research colony. We also thank Dr Gordon Pirie, Mr Sam Moran, and the BREC's Baton Rouge Zoo for supplying the macaws, and Ms Maryann Harris for supplying the African grey parrots.


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Hugues Beaufrere, DrMedVet, Mark Acierno, MBA, DVM, Dipl ACVIM, Mark Mitchell, DVM, MS, PhD, David Sanchez-Migallon Guzman, LV, MS, Dipl ECZM (Avian), Dipl ACZM, Heather Bryant, BS, and Thomas N. Tully Jr, DVM, MS, Dipl ABVP (Avian), Dipl ECZM (Avian)

From the Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Skip Bertman Dr, Baton Rouge, LA 70803, USA (Beaufrere, Acierno, Bryant, Tully); the Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois, 2001 S Lincoln Ave, Urbana, IL 61802, USA (Mitchell); and the Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, One Shields Ave, Davis, CA 95616, USA (Sanchez-Migallon Guzman).
Table 1. Plasma osmolality reference values in African grey parrots,
Hispaniolan Amazon parrots, and red-fronted macaws. Reference ranges
were determined by using the robust method.

                                    Mean                      range
         Species             n    (mOsm/kg)   SD   CV (%)   (mOsm/kg)

African grey parrot          21      306       7    2.3      288-324
Hispaniolan Amazon parrots   21      327       7    2.2      308-345
Red-fronted macaws            9      304      18    6.0      223-369

Abbreviations: n indicates sample size; CV, coefficient of variation.
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Author:Beaufrere, Hugues; Acierno, Mark; Mitchell, Mark; Guzman, David Sanchez-Migallon; Bryant, Heather; T
Publication:Journal of Avian Medicine and Surgery
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2011
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