this safe and inexpensive 4-aminoquinoline compound accumulates inside the digestive vacuole of the infected red blood cell . chloroquine resistance ( cqr ) was first reported in southeast asia and south america and has now spread to the vast majority of malaria - endemic countries . antimalarial drug resistance is a major public health problem which hinders the control of malaria . in india resistance of plasmodium falciparum to chloroquine was first reported in the year 1973 from diphu of the karbi anglong district in assam state . various in vitro sensitivity test systems have been developed and applied to sensitivity monitoring of p. falciparum in endemic areas . the most commonly used methods are in vitro tests based on the measurement of the effect of drugs on the growth and development of malaria parasites , that is , schizont maturation or growth inhibition [ 2 , 3 ] , incorporation of radiolabeled precursors , enzymatic activity of parasite lactate dehydrogenase ( pldh ) , or histidine - rich protein ii ( hrp ii ) . the in vitro sensitivity test based on the standard micro - technique recommended by the world health organization   using the schizont maturation inhibition test has been applied successfully in most of the highly multi - drug - resistant areas worldwide . the study was done to determine the prevalence of p. falciparum infection in aligarh , a north indian district and to test the chloroquine sensitivity using mark iii who sensitivity plates . the study was conducted in aligarh , a district of north india , where there is high prevalence of p. falciparum species . the study was conducted from february 2010 to december 2011 , but majority of cases ( > 90% ) were detected in months of july to october in both years . a total of 5653 patients , who presented with fever , chills , rigors , or any pyrexia of unknown origin were screened for malarial infection by using different diagnostic tests like peripheral blood smear examination and quantitative buffy coat assay and by various antigen detection assays . the drug sensitivity was done against chloroquine as it is the most commonly used antimalarial agent in out - patient departments for uncomplicated malarial infection . we also want to study chloroquine sensitivity because of its improper prescription and dosing in fever of unknown origin in this area by unqualified medical practitioners . 5  ml of blood was collected by venipuncture taking all sterile precaution in heparinized tube . both thick and thin smears were examined by staining with giemsa stained using standard protocols . parasitemia was determined by counting 200  wbcs on thick films and expressed as parasites per microliter of blood . a total of 200 oil emergence fields were examined before the smear is labeled negative or positive . the in vitro cultivation of p. falciparum isolates followed a modification of the standard culture techniques [ 7 , 8 ] . the culture medium consisted of rpmi 1640 ( sigma aldrich ) , 2  g glucose , and 40  g / ml gentamycin sulphate with supplemented 10% ab+ serum . culture medium was sterilised by filtration through a millipore filter of 0.22  m porosity and ph was adjusted to 7.4 by the addition of 4.2  ml of sterile 5% sodium bicarbonate . the drug sensitivity ( based on the m - iii methods ) can be determined in vitro in p. falciparum culture by using standard 96-well microtitre plates ( who plates ) , predosed of the test drug . h 0 , 1 , 2 , 4 , 8 , 16 , 32 , and 64  pmol . blood medium mixture ( bmm ) was prepared by gently shaking the tube to mix the blood and medium . all the wells of appropriate column were dosed with 50  l of the blood medium mixture ( 1  :  9 ) . the plate was put in a candle jar and placed in the incubator set at 37.5c for 2430  hrs , depending upon development stage of the parasite . after 24-hour incubation , a thin smear was prepared from the control well to see the mature schizonts , and if more than 10% schizonts were seen , it was considered to be valid ; thick smears were prepared from each well by discarding the excess media with a micropipette . the mean number of schizonts counts per well was fed directly into nonlinear regression software , h n - non - lin v1.1 , specific for malaria in vitro drug sensitivity test . individual dose response curves were generated and their ec 50 , ec 90 , and ec 99 values determined . the geometric means and 95% confidence intervals ( cis ) of ec values were estimated in spss ( spss inc . , usa ) . out of 5653 patients who presented with pyrexia of unknown origin or pyrexia associated with symptoms suggestive of malarial infection , only 435 patients had p. falciparum infection . fever was present in all patients ( 100% ) ; chills ( 85.51 ) , rigors ( 71.26% ) , and altered level of consciousness ( 60.74% ) were the common features with which p. falciparum positive cases presented ( table 1 ) . four hundred and thirty - five p. falciparum - positive blood samples were cultured ; 108 cultures were positive ; from all these isolates when subjected to chloroquine sensitivity , 26 isolates were found to be resistant to chloroquine , that is , schizont maturation at 8  pmols or more . isolates showed schizont maturation up to 32  pmols or more ( figure 1 ) . the mean effective concentrations of chloroquine ec 50 = 6.07  nm / liter , ec 90 = 33.33  nm / liter , ec 95 = 43.44  nm / liter , and ec 99 = 47.40  nm / liter , which shows decreased sensitivity of p. falciparum isolates to chloroquine ( table 2 ) . the major obstacle in malaria treatment and control is impeded by the drug - resistant parasites , particularly plasmodium falciparum , which have disseminated and enhanced mortality . chloroquine drug pressure remains high in india as it has been used since decades as first - line drug in malaria therapy both as self - treatment at home and in health - care facilities   shah et al . analyzed 337 studies and investigated chloroquine efficacy in 17189 patients . the number of studies and proportion of failures varied between regions and in states within a region . the median proportion of chloroquine failure was 35.1% ( iqr 13.058.2 ) in studies with a 28-day followup . studies with 7-day followup , which largely detected early treatment failures , were phased out from 2000 and the last 7-day study was done in 2003 . the proportion of failures detected is higher in 28-day follow - up studies than in 7-day follow - up studies done in the same areas , because late treatment failures are detected in 28-day follow - up studies . studies done between 1978 and 2007 show an increasing proportion of failures to chloroquine over time . when routine monitoring of drug resistance began in 1978 - 1979 , two of 17 studies exceeded the 10% threshold used in india to switch an area to the second - line treatment . in 2006 - 2007 , the proportion of chloroquine studies exceeding 10% treatment failure increased to 35 of 40 studies . drug - efficacy studies of chloroquine , with at least 30 patients in any follow - up period , have exceeded 10% treatment failure in 115 districts . the remaining states and territories have low incidence of p. falciparum , or have not done any trial of antimalarial drug resistance . only 31% cases have given adequate culture results due to low parasitemia , previous history of inadequate treatment by quacks . this is a tertiary care hospital where most of the patients come after getting treatment from one or more sources . most of the studies done in india mainly determined the in vivo resistance patterns of chloroquine . there is paucity of data from india regarding in vitro drug resistance pattern in india . in our study when chloroquine was tested against p. falciparum isolates , high level of resistance was noticed by in vitro mark iii sensitivity assay 53.76% of isolates resistant to 8  pmols of chloroquine which is the who cut - off value for sensitivity . the level of resistance in this part of india is so high that 10.75% strains were resistant to even 32  pmols of drug . one of the major objectives of the study is to determine effective concentration values ( ec values ) of chloroquine . our results showed that mean ec5o value of sensitive strains was 1.30 and for resistant 6.07 ( nmol / liter ) , ec 90 , ec 95 , and ec 99 were found to be 3.94  nm / liter and 33.33  nm / liter , 5.56  nm / liter and 43.44  nm / liter , 6.17  nm / liter , and 47.40  nm / liter for sensitive and resistant isolates , respectively . these results indicate high level of resistance of p. falciparum to chloroquine among the isolates tested , and this is confirmed by high ec values which are excessively higher than effective therapeutic concentration . these values are significantly higher than those reported from india and other countries [ 13 , 14 ] . this high - level resistance to chloroquine in our patients may be due to previous exposure to improper therapeutic regimes , over - the - counter availability of drug , and high drug pressure by improper prescribing habits of private practitioners . so this high level of resistance should be monitored regularly to prevent further spread of resistant strains . the use of combination therapy should be advocated to prevent the drug resistance in malaria - endemic areas .