Simple Investigations


This blood test is the simplest biochemical measure of assessing renal function.

– By looking at the plasma concentration of creatinine and urea, this allows you to estimate the glomerular filtration rate (GFR) to see if the kidneys are effectively filtering the blood.

a) Serum Creatinine:

This is a breakdown product of creatine phosphate in skeletal muscle –> filtered, not reabsorbed and slowly secreted (gives 15% overestimate)

– Therefore, GFR = rate of creatine excretion/plasma creatinine concentration.

– As creatinine is proportional to muscle mass, the result requires adjustment using the Modification of Diet in Renal Disease (MDRD) equation to calculate the effective GFR (eGFR)

– It uses the following variables: Serum creatinine level + Age + Gender + Ethnicity

– Affected by pregnancy + Muscle mass + eating red meat 12 hours prior to sample being taken

b) Serum Urea:

Urea is a waste product in that is formed from the breakdown of proteins

– It is usually excreted by the kidneys, so a decrease in kidney function will cause urea levels to rise

– Although most commonly used to assess renal function, levels of urea may also indicate other pathologies.  The serum urea:creatinine ratio can be used to tell the origin of an acute kidney injury.

Raised Urea levels:

– Kidney disease/failure

– Higher protein levels

– Organ damage (e.g. heart attack)

– Stress

Lower Urea levels:

– Liver failure

– Malnutrition

– Lack of protein in diet

Urine dipstick

This is performed whenever there is a suspicion of a pathology affecting the urinary tract.

– It is a cheap, quick and convenient method of checking the constituents of the urine.

– There are several different things that a urine analysis can indicate. For all tests, it is necessary to wait 60s before analysis, except leukocytes where it is advised to wait 2 minutes as it tests enzyme activity.


This indicates the acidity of the urine. This will be decreased in a systemic acidosis

b) Blood cells [RENAL FUNCTION]:

It is abnormal to detect blood in the urine (haematuria).

– If present this is classified as Visible (VH – macroscopic) or non-visible (microscopic). This is divided into symptomatic or asymptomatic.


– Malignancy

– Renal stones


– Strenuous exercise, due to myoglobin in urine

–> Myoglobin triggers the same dipstick reaction which can give a false positive result.

c) Protein [RENAL FUNCTION]:

This indicates the levels of protein in the urine and requires quantification.

– This is done using the albumin: creatinine ratio – normal A:CR is <2.5 (male) or <3.5 (female)

– It is abnormal to detect protein in urine, so its presence suggests glomerular disease


Glomerulonephritis, DM, amyloidosis, nephrotic syndrome

False Positives:

Fever, exercise, heart failure (due to troponin) and postural

d) Leukocytes [INFECTION]:

This test is for the presence of leukocyte esterase, an enzyme made by neutrophils in urine

– The presence of white blood cells (WBC’s) in the urine is suggestive that your body is trying to fight a pathogen, raising the suspicion of a urinary infection.

– If there is suspicion of infection, take urine cultures and microscopy

e) Nitrites [INFECTION]:

This tests for nitrites which are made by nitrate-reducing bacteria.

– This test is a quick screening test for the presence of gut bacteria in the urine. However, it is not diagnostic of a UTI and would require culture for confirmation.

f) Urobilinogen [SPECFIC TEST]:

This is formed by the breakdown and reduction of bilirubin

– A higher than normal urobilogen suggests a possible deterioration of liver functions e.g. liver cirrhosis, hepatitis + conditions associated with haemolytic anaemia.

g) Glucose [SPECFIC TEST]:

Normally glucose is completely reabsorbed. Its presence indicates diabetes/renal glycosuria

h) Ketones [SPECFIC TEST]:

Suggests starvation/diabetes causing ketone production

i) Specific Gravity [SPECFIC TEST]:

This test shows how much solute is dissolved in urine.

– A high gravity is a result of dehydration, which may occur when people are not drinking enough or losing much water e.g. Diabetes Insipidus.

– Normal range is 1.020-1.030

Fluid balance

It is essential to balance the normal fluid intake against losses to prevent against dehydration.

– Normal intake is about 2.5l from food and drink. This is balances against loss from the urine (1.5l), the faeces (300ml) and sweating (700ml)


There are many different signs of an abnormal fluid balance in the body.

Underfilled: (like hypovolemic shock)

– Tachycardia

– Low Blood pressure

– Slow capillary refill time

– Low urine output

Overfilled: (like heart failure)

– Raised JVP

– Pitting edema

– Bibasal crackles

– Pulmonary edema

There are many different types of IV fluids – when deciding which one to prescribe it is essential to consider the osmolality, as this will determine how quickly and where the fluid spreads to:

0.9% saline:

This is isotonic with plasma and has the same Na+ content as plasma (150mM)

– This equilibrates rapidly through the extracellular content and takes longer to reach the intracellular compartment.

– You can add potassium separately by giving Sando-K (contains 12mM K+) and also Cl-

– It is appropriate for fluid resuscitation, as remains in extracellular space

– Maintenance fluid rate = 30ml/kg/24hr

Side Effects

Can cause a hyperchloremic metabolic acidosis

Hartmann’s solution:

This contains Na+ 131 mM, Cl- 111mM and some anions like lactate

– It is used as an alternative to 0.9% saline.

– For maintenance we need 1mM of K+/Na+/Cl- per kg per 24 hrs + 50-100g/day glucose

5% glucose/dextrose:

This is isotonic in theory, so should act like 0.9% saline.

– However, the glucose is quickly used up decreasing the osmolality of the solution

– The water left then distributes though all the body fluid compartments, so only 1/12 will remain in blood

– This means that it is used for more as a maintenance fluid, not suitable for fluid resuscitation.

Side Effects

Can lead to hyponatreamia –> cerebral oedema, headaches, raised ICP

Colloids (Gelufosine):

These have a high osmotic content which means that they stay in the blood

– This means they are good at expanding the plasma volume, so can be used in resuscitation


These agents are hypertonic and aim to draw water out of the intracellular space

– They are used in cerebral oedema to reduce intracranial pressure in emergencies.

Side Effects

Hypersensitivity reactions


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